Zoom lens, and electronic imaging system using the same

ABSTRACT

A zoom lens includes a first lens group that remains fixed during zooming, a second lens group having a negative refractive power that moves during zooming, a third lens group that has a positive refractive power and moves during zooming, and a fourth lens group that has a positive refractive power and moves during zooming and focusing. The first lens group includes a negative meniscus convex lens component, a reflecting optical element for bending an optical path, and a positive lens. The first lens group may include a reflecting optical element that upon focusing on an infinite object point, the fourth group lens moves in a locus opposite to the movement of the third lens group during zooming.

This application claims benefits of Japanese Application No. 2002-106378filed in Japan on Apr. 9, 2002, the contents of which are incorporatedherein by this reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to a zoom lens and an electronicimaging system using the same, and more particularly to an electronicimaging system such as a video camera or a digital camera, the depthdimension of which is diminished by providing some contrivances to anoptical system portion such as a zoom lens.

In recent years, digital cameras (electronic cameras) have receivedattention as the coming generation of cameras, an alternative tosilver-halide 35 mm-film (usually called Leica format) cameras.Currently available digital cameras are broken down into some categoriesin a wide range from the high-end type for commercial use to theportable low-end type.

In view of the category of the portable low-end type in particular, theprimary object of the present invention is to provide the technology forimplementing video or digital cameras whose depth dimension is reducedwhile high image quality is ensured, and which are easy to handle.

The gravest bottleneck in diminishing the depth dimension of cameras isthe thickness of an optical system, especially a zoom lens system fromthe surface located nearest to its object side to an image pickup plane.

Recent technologies for slimming down cameras rely primarily on acollapsible lens mount that allows the optical system to be taken out ofa camera body for phototaking and received therein for carrying. Typicalexamples of an optical system that can effectively be slimmed down whilerelying on the collapsible lens mount are disclosed in JP-A's 11-194274,11-287953 and 2000-9997. Each publication discloses an optical systemcomprising, in order from its object side, a first lens group havingnegative refracting power and a second lens group having positiverefracting power, wherein both lens groups move during zooming.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided azoom lens, comprising, in order from its object side, a first lens groupthat remains fixed during zooming, a second lens group that has negativerefracting power and moves during zooming, a third lens group that haspositive refracting power and moves during zooming, and a fourth lensgroup that has positive refracting power and moves during zooming andfocusing, characterized in that the first lens group comprises, in orderfrom its object side, a negative meniscus lens component convex on itsobject side, a reflecting optical element for bending an optical pathand a positive lens.

According to another aspect of the present invention, there is provideda zoom lens, comprising, in order from its object side, a first lensgroup that remains fixed during zooming, a second lens group that hasnegative refracting power and moves during zooming, a third lens groupthat has positive refracting power and moves during zooming, and afourth lens group that has positive refracting power and moves duringzooming and focusing, characterized in that the first lens groupcomprises a reflecting optical element for bending an optical path, andupon focusing on an infinite object point, the fourth lens group movesin a locus opposite to that of movement of the third lens group duringzooming.

The advantages of, and the requirements, for the above arrangements usedherein are now explained.

While relying upon the arrangement comprising, in order from its objectside, the first lens group that remains fixed during zooming, the secondlens group that has negative refracting power and moves during zooming,the third lens group that has positive refracting power and moves duringzooming and the fourth lens group that has positive refracting power andmoves during both zooming and focusing, the zoom lens of the presentinvention enables an associated camera to be immediately put into theready state unlike a collapsible lens mount camera. To be favorable forwater-proofing and dust-proofing purposes, the first lens group isdesigned to remain during zooming, and for considerably reducing thedepth dimension of the camera, at least one reflecting optical elementfor bending an optical path is located in the first lens group nearestto the object side of the lens system.

However, the location of the optical path-bending reflecting opticalelement in the first lens group would give rise to the following twodemerits.

A. The depth of an entrance pupil increases, leading unavoidably to anincrease in the size of each lens element forming the first lens groupthat, by definition, has a large diameter.

B. The magnification of a combined system comprising the second or thethird lens group that, by definition, has a zooming function and thesubsequent lens group or groups is close to zero, and so the zoom ratiobecomes low relative to the amount of zooming movement.

First of all, the condition necessary for bending is explained.Referring to a zoom type such as one intended herein, the location ofthe optical path-bending reflecting optical element in the first lensgroup necessarily makes the position of the entrance pupil likely tobecome deep, as in the case of JP-A 10-62687 or 11-258507, resulting inan increase in the size of each optical element that forms the firstlens group. It is thus preferable that the first lens group comprises,in order from its object side, a negative meniscus lens component convexon its object side, a reflecting optical element for bending an opticalpath and a positive lens and satisfies the following conditions (1),(2), (3) and (4).

1.4<−f ₁₁/{square root over ( )}(f _(W) ·f _(T))<2.4  (1)

1.2<f ₁₂/{square root over ( )}(f _(W) ·f _(T))<2.2  (2)

0.8<d/L<2.0  (3)

1.55<n _(PRI)  (4)

Here f₁₁ is the focal length of the negative meniscus lens component inthe first lens group, f₁₂ is the focal length of the positive lenselement in the first lens group, f_(W) and f_(T) are the focal lengthsof the zoom lens at the wide-angle end and the telephoto end of the zoomlens, respectively, d is an air-based length from the image side-surfaceof the negative meniscus lens component to the object side-surface ofthe positive lens element in the first lens group, as measured on theoptical axis of the zoom lens, L is the diagonal length of the(substantially rectangular) effective image pickup area of an electronicimage pickup device, and N_(PRI) is the d-line refractive index of themedium of a prism used as the optical path-bending reflecting opticalelement in the first lens group.

In order to locate the entrance pupil at a shallow position therebyenabling the optical path to be physically bent, it is preferable toincrease the powers of the lens elements on both sides of the first lensgroup, as defined by conditions (1) and (2). As the upper limits of 2.4and 2.2 to both conditions are exceeded, the entrance pupil remains at adeep position. Hence, when it is intended to ensure some angle of view,the diameter or size of each optical element forming the first lensgroup becomes too large to physically bend the optical path. As thelower limits of 1.4 and 1.2 are not reached, the magnification that thelens groups subsequent to the first lens group and designed to move forzooming can have becomes close to zero, offering problems such as anincrease in the amount of zooming movement or a zoom ratio drop and, atthe same time, rendering correction of off-axis aberrations such asdistortion and chromatic aberrations difficult.

Condition (3) is provided to determine the length necessary for thelocation of the optical path-bending reflecting optical element, asmeasured along the optical axis of the zoom lens. Although the value ofthis condition should preferably be as small as possible, it isunderstood that as the lower limit of 0.8 thereto is not reached, alight beam contributing to the formation of an image at the periphery ofa screen does not satisfactorily arrive at the image plane or ghosts arelikely to occur. As the upper limit of 2.0 is exceeded, it is physicallydifficult to bend the optical path as in the case of conditions (1) and(2).

As can be understood from the foregoing, the air-based length, d, asdefined by condition (3) should preferably be cut down by using as theoptical path-bending element in the first lens group a prism in whichentrance and exit surfaces are formed of planar surfaces or different incurvature from the lens surfaces on both sides of the first lens groupand making the refractive index of a prism medium as high as possible.As the lower limit of 1.55 to condition (4) is not reached, it isphysically difficult to bend the optical path. It is also preferablethat n_(PRI) does not exceed 1.90. Exceeding 1.90 means that the prismcosts much, and renders ghosts likely to occur.

More preferably, at least one or all of the following conditions (1)′,(2)′, (3)′ and (4)′ should be satisfied.

1.5<−f ₁₁/{square root over ( )}(f _(W) ·f _(T))<2.2  (1)′

1.3<f ₁₂/{square root over ( )}(f _(W) ·f _(T))<2.0  (2)′

0.9<d/L<1.7  (3)′

1.65<n_(PRI)  (4)′

Even more preferably, at least one of the following conditions (1)″,(2)″, (3)″ and (4)″ should be satisfied.

 1.6<−f ₁₁/{square root over ( )}(f _(W) ·f _(T))<2.0  (1)″

1.4<f ₁₂/{square root over ( )}(f _(W) ·f _(T))<1.8  (2)″

1.0<d/L<1.5  (3)″

1.75<n_(PRI)  (4)″

Most preferably, all conditions (1)″ to (4)″ should be satisfied.

Further, the zoom lens of the present invention should preferablysatisfy the following condition (a).

1.8<f _(T) /f _(W)  (a)

Here f_(W) is the focal length of the zoom lens at the wide-angle end,and f_(T) is the focal length of the zoom lens at the telephoto end.

Falling short of the lower limit of 1.8 to this condition means that thezoom ratio of the zoom lens becomes lower than 1.8. More preferably inthis case, the value of f_(T)/f_(W) should not exceed 5.5. At greaterthan 5.5, the zoom ratio becomes high and the amount of the lens groupsthat move during zooming becomes too large. This causes the zoom lens tobecome large in the optical path-bending direction, and so renders itimpossible to set up any compact imaging system.

Next, how to ensure any desired zoom ratio is explained. When the firstlens group of the present invention has positive refracting power, theposition of a principal point is evidently located nearer to the imageside of the zoom lens as compared with the case where there is nooptical path-bending reflecting optical element. This means that withthe same refracting power, the position of an image point by the firstlens group is located nearer to the image side; that is, an object pointwith respect to the second lens group is located at a farther position.Consequently, the magnification of the second lens group approacheszero, and the change in the focal length of the zoom lens becomes smalleven upon movement of the second lens group. One approach to solvingthis problem is to make the focal length of the first lens group short(so that the focal length of the zoom lens becomes short), whereby thefocal length of the second lens group is increased to a certain degreeto increase the magnification of the second lens group. According to thepresent invention wherein a combined system comprising the third lensgroup and the subsequent lens group (or groups) is also allowed to havea zooming function, if the magnifications, zoom ratios, etc. of both areartfully set, it is then possible to provide an efficient zooming of thezoom lens. Specific conditions to this end are determined by thefollowing conditions (5), (6) and (7).

0.4<−β_(2W)<1.2  (5)

0.1<−β_(RW)<0.5  (6)

0<log γ_(R)/log γ₂<1.3  (7)

Here β_(2W) is the magnification of the second lens group at thewide-angle end of the zoom lens upon focused on an infinite objectpoint, β_(RW) is the composite magnification of a combined systemcomprising the third lens group and all subsequent lens groups at thewide-angle end upon focused on an infinite object point, γ₂ isβ_(2T)/β_(2W) provided that β_(2T) is the magnification of the secondlens group at the telephoto end of the zoom lens upon focused on aninfinite object point, and γ_(R) is β_(RT)/β_(RW) provided that β_(RT)is the composite magnification of a combined system comprising the thirdlens group and all subsequent lens groups at the telephoto end uponfocused on an infinite object point.

As the lower limits of 0.4 and 0.1 to conditions (5) and (6) are notreached, any satisfactorily high zoom ratio cannot be obtainedthroughout the zoom lens or the moving space becomes too large,resulting in a size increase. This in turn renders correction of variousaberrations difficult, partly because the focal length of the first lensgroup becomes too short, and partly because the Petzval sum becomeslarge. Exceeding the upper limit of 1.3 to condition (7) is notpreferable because fluctuations of the F-number and exit pupil positionwith zooming become too large. As the lower limit of 0 is not reached,the entrance pupil becomes too deep and the bending of the optical pathbecomes physically difficult. In any case, any satisfactorily high zoomratio cannot be obtained throughout the zoom lens, or the moving spacebecomes too large, leading to a bulky size.

More preferably, at least one or all of the following conditions (5)′,(6)′ and (7)′ should be satisfied.

0.4<−β_(2W)<1.1  (5)′

0.20<−β_(RW)<0.45  (6)′

0.15<log γ_(R)/log γ₂<1.2  (7)′

Even more preferably, at least one of the following conditions (5)″,(6)″ and (7)″ should be satisfied.

0.6<−β_(2W)<1.0  (5)″

0.25<−β_(RW)<0.4  (6)″

0.25<log γ_(R)/log γ₂<1.0  (7)″

In order to satisfy conditions (5), (6) and (7), the followingconditions (8) and (9), too, should preferably be satisfied.

1.6<f ₁/{square root over ( )}(f _(W) ·f _(T))<6.0  (8)

1.1<−f ₂/{square root over ( )}(f _(W) ·f _(T))<2.2  (9)

Here f₁ is the focal length of the first lens group, f₂ is the focallength of the second lens group, and f_(W) and f_(T) are the focallengths of the zoom lens at the wide-angle and the telephoto end,respectively.

As the upper limit to condition (8) is exceeded, any satisfactorily highmagnification cannot be obtained throughout the zoom lens or the movingspace becomes too large, leading to a bulky size. As the lower limit isnot reached, correction of off-axis aberrations and chromaticaberrations becomes difficult.

As the upper limit of 2.2 to condition (9) is exceeded, zoomingefficiency becomes high thanks to an increase in the magnification ofthe second lens group; however, the efficiency may rather decreasebecause the amount of movement to obtain the same zoom ratio isproportional to the focal length. As the lower limit of 1.1 is notreached, the magnification of the second lens group comes close to zero,ending up with a zooming efficiency drop.

More preferably, the following conditions (8)′ and/or (9)′ should besatisfied.

1.9<f ₁/{square root over ( )}(f _(W) ·f _(T))<4.5  (8)′

1.2<−f ₂/{square root over ( )}(f _(W) ·f _(T))<2.0  (9)

Even more preferably, the following conditions (8)″ and/or (9)″ shouldbe satisfied.

2.2<f ₁/{square root over ( )}(f _(W) ·f _(T))<3.0  (8)″

1.3<−f ₂/{square root over ( )}(f _(W) ·f _(T))<1.8  (9)″

Most preferably, both conditions (8)″ and (9)″ should be satisfied.

As the second lens group is designed with a high magnification, anotherproblem arises. The magnification of the second lens group becoming highmeans that an object point with respect to a combined system thatcomprises the third lens group and the subsequent lens group or groupsand has another zooming function is located at a farther position andthe magnification of the combined system comes close to zero, resultingin a drop of zooming efficiency by that combined system. There are twoapproaches to solving this problem; one is to make the focal length ofthe combined system comprising the third lens group and the subsequentlens group or groups long to a certain degree, and another is to bring aprincipal point as close to an image point for the second lens group aspossible. In the former case, the following condition (10) shouldpreferably be satisfied.

0.8<f _(RW)/{square root over ( )}(f _(W) ·f _(T))<1.7  (10)

Here f_(RW) is the composite focal length of the combined systemcomprising the third lens group and the subsequent lens group or groups,and f_(W) and f_(T) are the focal lengths of the zoom lens at thewide-angle and the telephoto end, respectively.

As the lower limit of 0.8 to condition (10) is not reached, the zoomingefficiency by the combined system comprising the third lens group andthe subsequent lens group or groups becomes worse. As the upper limit of1.7 is exceeded, the zooming efficiency becomes worse for the samereason as in condition (9). In the latter case, the third lens groupshould preferably have therein at least one converging surface that isdefined by an air contact surface convex on its object side andsatisfies the following condition (b) and at least one diverging surfacethat is located on an image side with respect to the converging surface,is defined by an air contact surface convex on its image side andsatisfies the following condition (b).

0<R _(P) /f _(W)<2  (b)

0<R _(N) /f _(W)<4  (c)

Here R_(P) and R_(N) are the axial radii of curvature of the convergingsurface and the diverging surface, respectively. Otherwise, it isdifficult to bring the principal point for the third lens group close tothe image point for the second lens group.

More preferably, the following condition (10)′ should be satisfied.

0.9<f _(RW)/{square root over ( )}(f _(W) ·f _(T))<1.5  (10)′

Most preferably, the following condition (10)″ should be satisfied.

1.0<f _(RW)/{square root over ( )}(f _(W) ·f _(T))<1.3  (10)″

Particularly preferably for both the cases, the focal length of thecombined system comprising the third lens group and the subsequent lensgroup or groups should be increased upon zooming from the wide-angle endto the telephoto end, as defined by the following condition (11).

1.0<f _(RT) /f _(RW)<2.5  (11)

Here f_(RW) is the composite focal length of the combined systemcomprising the third lens group and all the subsequent lens groups atthe wide-angle end, and f_(RT) is the composite focal length of thecombined system comprising the third lens group and all the subsequentlens groups at the telephoto end.

As the lower limit of 1.0 to condition (11) is not reached, the effectof the combined system comprising the third and subsequent lens groupson zooming becomes slender, the amount of movement of the second lensgroup increases and the entrance pupil becomes deep, and it is difficultto bend the optical path. As the upper limit of 2.5 is exceeded,fluctuations of F-number with zooming tend to become noticeable.

More preferably, the following condition (11)′ should be satisfied.

1.1<f _(RT) /f _(RW)<2.3  (11)′

Most preferably, the following condition (11)″ should be satisfied.

1.2<f _(RT) /f _(RW)<2.1  (11)″

According to the method most effective for the achievement of condition(11), the third lens group that, by definition, must be located as closeto the image plane as possible at the wide-angle end with a view toobtaining high zoom ratios and the lens group located nearest to theobject side of the zoom lens in the subsequent lens groups (hereinaftercalled the fourth lens group) should rather be located as near to theobject side as possible, so that upon zooming to the telephoto side, thethird lens group is moved toward the object side while the fourth lensgroup is moved toward the image side of the zoom lens (upon focusing onan infinite object point).

Specific conditions to this end are to satisfy the following conditions(12) and (13).

0.20<−M ₃ /M ₂<1.50  (12)

0.15<−M ₄ /M ₃<1.00  (13)

where M₂ is the amount of movement of the second lens group from thewide-angle end to the telephoto end, M₃ is the amount of movement of thethird lens group from the wide-angle end to the telephoto end, and M₄ isthe amount of movement of the fourth lens group from the wide-angle endto the telephoto end, provided that the movement of each lens grouptoward the image side is of positive sign.

Exceeding the upper limit of 1.50 to condition (12) is not preferablebecause fluctuations of F-number and an exit pupil position with zoomingbecome too noticeable. As the lower limit of 0.20 is not reached, theentrance pupil becomes too deep and so it is physically hard to bend theoptical path. In any case, any satisfactorily high zoom ratio cannot beobtained throughout the zoom lens or the moving space becomes too large,leading to a bulky size.

As the upper limit of 1.00 to condition (13) is exceeded, themagnification of the combined system comprising the third and subsequentlens groups may become high. Since a main moving lens group is thefourth lens group for focusing, however, this is not preferable becausefluctuations of magnification with focusing tend to become noticeable.As the lower limit of 0.15 is not reached, the principal point for thecombined system comprising the third and subsequent lens groups is faraway from the image point for the second lens group. This in turn causesa drop of zooming efficiency, or renders the focal length of thecombined system comprising the third and subsequent lens group or groupslikely to become long or the lens arrangement of the third andsubsequent lens group or groups unreasonable, offering an obstacle tocorrection of aberrations.

More preferably, the following conditions (12)′ and/or (13)′ should besatisfied.

0.30<−M ₃ /M ₂<1.40  (12)′

0.20<−M ₄ /M ₃<0.80  (13)′

Even move preferably, the following condition (12)″ or (13)″ should besatisfied.

0.40<−M ₃ /M ₂<1.30  (12)″

0.25<−M ₄ /M ₃<0.60  (13)″

Most preferably, both conditions (12)″ and (13)″ should be satisfied.

It is noted that focusing should preferably be performed with the fourthlens group. It is then preferable to satisfy the following condition(14).

0.10<D _(34W) /f _(W)<0.70  (14)

Here D_(34W) is an air separation between the third lens group and thefourth lens group at the wide-angle end upon focused on an infiniteobject point, and f_(W) is the focal length of the zoom lens at thewide-angle end.

As the lower limit of 0.10 to this condition is not reached, the thirdlens group is prone to interference with the fourth lens group for lackof any focusing space. As the upper limit of 0.70 is exceeded,conversely, the moving space for zooming tends to become insufficient.

More preferably,

0.15<D _(34W) /f _(W)<0.60  (14)′

Most preferably,

0.20<D _(34W) /f _(W)<0.50  (14)″

When focusing is performed by movement of the fourth lens group, on theother hand, astigmatism tends generally to be placed in an ill-balancedstate. This astigmatism is likely to occur especially when residualastigmatism occurring at the 1st to 3rd lens groups is corrected at thefourth lens group. Thus, both refracting surfaces of any one of the lenscomponents forming the third lens group, inclusive of the doubletcomponent, should be configured as aspheric surfaces. It is alsopreferable to incorporate at least one doublet component of a positiveand a negative lens element in the third lens group because chromaticaberrations should preferably be corrected at the third lens group thathas generally high light rays. It is understood that the “lenscomponent” used herein means a lens that contacts spaces on both sidesalone and has any air contact surface nowhere on the optical path, e.g.,a single lens or a doublet.

The construction of the third lens group is now explained in detail.

The third lens group may be made up of, in order from its object side:

1) a doublet component consisting of a positive lens element and anegative lens element and a single lens element configured as sphericalsurfaces at both surfaces, two subgroups or three lens elements in all,

2) a doublet component consisting of a single lens element configured asaspheric surfaces at both surfaces and a doublet component consisting ofa positive lens element and a negative lens element, two subgroups orthree lens elements in all, or

3) only a doublet component consisting of a positive lens elementconfigured as aspheric surfaces at both air contact surfaces and anegative lens element, one group or two lens elements in all.

In any case, the doublet component may serve to slack the relativedecentration sensitivity between the lens elements that form the thirdlens group.

Corresponding to the types 1), 2) and 3) of the third lens group, it isfurther preferable to satisfy the following conditions (15-1), (15-2)and (15-3), respectively (with respect to correction of aberrations andslacking of decentration sensitivity).

1.05<R _(C3) /R _(C1)<3.00  (15-1)

0.25<R _(C3) /R _(C1)<0.75  (15-2)

1.20<R _(C3) /R _(C1)<3.60  (15-3)

Here R_(C1) is the axial radius of curvature of the surface nearest tothe object side of the doublet component, and R_(C3) is the axial radiusof curvature of the surface nearest to the image side of the doubletcomponent.

Exceeding the respective upper limits of 3.00, 0.75 and 3.60 to theseconditions (15-1), (15-2) and (15-3) may be favorable for correction ofspherical aberrations, coma and astigmatism throughout the zoom lens;however, the effect of cementing on slacking of decentration sensitivitybecomes slender. As the respective lower limits of 1.05, 0.25 and 1.20are not reached, correction of spherical aberrations, coma andastigmatism throughout the zoom lens becomes difficult.

More preferably,

1.15<R _(C3) /R _(C1)<2.50  (15-1)′

0.30<R _(C3) /R _(C1)<0.65  (15-2)′

1.40<R _(C3) /R _(C1)<3.00  (15-3)′

Most preferably,

1.25<R _(C3) /R _(C1)<2.00  (15-1)″

0.35<R _(C3) /R _(C1)<0.55  (15-2)″

1.60<R _(C3) /R _(C1)<2.40  (15-3)″

Furthermore corresponding to the types 1), 2) and 3) of the third lensgroup, it is preferable to satisfy the following conditions (16-1) and(17-1), (16-2) and (17-2), and (16-3) and (17-3) with respect tocorrection of chromatic aberrations.

−0.7<L/R _(C2)<0.1  (16-1)

10<ν_(CP)−ν_(CN)  (17-1)

 −0.5<L/R _(C2)<0.3  (16-2)

20<ν_(CP)−ν_(CN)  (17-2)

−0.9<L/R _(C2)<−0.1  (16-3)

10<ν_(CP)−ν_(CN)  (17-3)

Here L is the diagonal length in mm of an effective image pickup area ofthe electronic image pickup device, R_(C2) is the axial radius ofcurvature of a cementing surface of the doublet component in the thirdlens group, ν_(CP) is the d-line based Abbe number of a medium of thepositive lens element of the doublet component in the third lens group,and ν_(CN) is the d-line based Abbe number of a medium of the negativelens element of the doublet component in the third lens group with theproviso that the electronic image pickup device is used in such a way asto include an angle of view of 55° or greater at the wide-angle end.

Falling short of the respective lower limits of −0.7, −0.5 and −0.9 toconditions (16-1), (16-2) and (16-3) may be favorable for correction oflongitudinal chromatic aberration and chromatic aberration ofmagnification; however, this is not preferable because chromaticaberration of spherical aberration is likely to occur, and sphericalaberrations at short wavelengths remain over-corrected even whenspherical aberrations at the reference wavelength can be well corrected,causing chromatic blurring of images. As the respective upper limits of0.1, 0.3 and −0.1 are exceeded, correction of longitudinal chromaticaberration and chromatic aberration of magnification tends to becomeinsufficient and spherical aberrations at short wavelengths are prone tounder-correction.

As the respective lower limits of 10, 20 and 10 to conditions (17-1),(17-2) and (17-3) are not reached, correction of longitudinal chromaticaberration tends to become insufficient. The upper limits to conditions(17-1), (17-2) and (17-3) may prima facie be set at 90. Any combinationsof media exceeding the upper limit of 90 do not occur in nature. Apreferable upper limit to ν_(CP)−ν_(CN) is 60. Materials of greater than60 are expensive.

More preferably, either one or both of the following conditions (16-1)′and (17-1)′, (16-2)′ and (17-2)′, and (16-3)′ and (17-3)′ should besatisfied.

−0.6<L/R _(C2)<0.0  (16-1)′

15<ν_(CP)−ν_(CN)  (17-1)′

−0.4<L/R _(C2)<0.2  (16-2)′

25<ν_(CP)−ν_(CN)  (17-2)′

−0.8<L/R _(C2)<−0.2  (16-3)′

15<ν_(CP)−ν_(CN)  (17-3)′

Even more preferably, either one of the following conditions (16-1)″ and(17-1)″, (16-2)″ and (17-2)″, and (16-3)″ and (17-3)″ should besatisfied.

−0.5<L/R _(C2)<−0.1  (16-1)″

20<ν_(CP)−ν_(CN)  (17-1)″

 −0.3<L/R _(C2)<0.1  (16-2)″

30<ν_(CP)−ν_(CN)  (17-2)″

−0.7<L/R _(C2)<−0.3  (16-3)″

20<ν_(CP)−ν_(CN)  (17-3)″

Most preferably, both of the above conditions (16-1)″ and (17-1)″,(16-2)″ and (17-2)″, and (16-3)″ and (17-3)″ should be satisfied.

The fourth lens group should preferably be composed of one positive lenscomponent and satisfy the following conditions (18) and (19).

−4.00<(R _(4F) +R _(4R))/(R _(4F) −R _(4R))<0.0  (18)

0.10<L/f ₄<0.70  (19)

Here R_(4F) is the axial radius of curvature of the object side-surfaceof the positive lens component, R_(4R) is the axial radius of curvatureof the image side-surface of the positive lens component, L is thediagonal length of an effective image pickup area of the electronicimage pickup device, and f₄ is the focal length of the fourth lensgroup.

Exceeding the upper limit of 0.0 to condition (18) is not preferable forzooming efficiency because a principal point for the combined systemcomprising the third and subsequent lens groups tends to be far awayfrom the image point by the second lens group. As the lower limit of−4.00 is not reached, fluctuations of astigmatism with focusing tend tobecome large.

As the upper limit of 0.70 to condition (19) is exceeded, the third andfourth lens groups cannot move in opposite directions during zooming.Falling short of the lower limit of 0.10 is not preferable because theamount of movement of the fourth lens group for focusing becomes toolarge.

More preferably, either one or both of the following conditions (18)′and (19)′ should be satisfied.

−3.60<(R _(4F) +R _(4R))/(R _(4F) −R _(4R))<−0.4  (18)′

0.15<L/f ₄<0.60  (19)′

Even more preferably, either one of the following conditions (18)″ and(19)″ should be satisfied.

−3.20<(R _(4F) +R _(4R))/(R _(4F) −R _(4R))<−0.80  (18)′

0.20<L/f ₄<0.50  (19)′

Most preferably, both of the above conditions (18)″ and (19)″ should besatisfied.

For the second lens group having a long focal length, it should be onlycomposed of, in order from its object side, a negative lens element anda positive lens element, two lens elements in all. In conjunction withthe first lens group, it is preferable to satisfy the followingconditions (20) and (21).

−0.80<(R _(1PF) +R _(1PR))/(R _(1PF) −R _(1PR))<0.90  (20)

−0.10<(R _(2NF) +R _(2NR))/(R _(2NF) −R _(2NR))<2.00  (21)

Here R_(1PF) is the axial radius of curvature of the object side-surfaceof the positive lens component in the first lens group, R_(1PR) is theaxial radius of curvature of the image side-surface of the positivecomponent in the first lens group, R_(2NF) is the axial radius ofcurvature of the object side-surface of the negative lens component inthe second lens group, and R_(2NR) is the axial radius of curvature ofthe image side-surface of the negative lens component in the second lensgroup.

As the upper limit of 0.90 to condition (20) is exceeded, higher-orderchromatic aberrations of magnification tend to occur, and as the lowerlimit of −0.80 is not reached, the entrance pupil tends to become deep.

As the upper limit of 2.00 to condition (20) is exceeded, coma tends tooccur, and as the lower limit of −0.10 is not reached, barrel distortiontends to occur.

More preferably, either one or both of the following conditions (20)′and (21)′ should be satisfied.

−0.50<(R _(1PF) +R _(1PR))/(R _(1PF) −R _(1PR))<0.70  (20)′

−0.20<(R _(2NF) +R _(2NR))/(R _(2NF) −R _(2NR))<1.50  (21)′

Even more preferably, either one of the following conditions (20)″ and(21)″ should be satisfied.

−0.20<(R _(1PF) +R _(1PF))/(R _(1PF) −R _(1PR)<0.50  (20)″

0.50<(R _(2NF) +R _(2NR))/(R _(2NF) −R _(2NR))<1.50  (21)″

Most preferably, both of the above conditions (20)″ and (21)″ should besatisfied.

The presumption for the electronic image pickup device used herein isthat it has a total angle of view of 55° or greater at the wide-angleend. The 55 degrees are the wide-angle-end total angle of view neededcommonly for electronic image pickup devices.

For the electronic image pickup device, the wide-angle-end total angleof view should preferably be 80° or smaller. At greater than 80°,distortions are likely to occur, and it is difficult to make the firstlens group compact. It is thus difficult to slim down the electronicimaging system.

Thus, the present invention provides means for reducing the thickness ofthe zoom lens portion while satisfactory image-formation capability ismaintained.

Next, how and why the thickness of filters is reduced is now explained.In an electronic imaging system, an infrared absorption filter having acertain thickness is usually inserted between an image pickup device andthe object side of a zoom lens, so that the incidence of infrared lighton the image pickup plane is prevented. Here consider the case wherethis filter is replaced by a coating devoid of thickness. In addition tothe fact that the system becomes thin as a matter of course, there arespillover effects. When a near-infrared sharp cut coat having atransmittance (τ₆₀₀) of at least 80% at 600 nm and a transmittance(τ₇₀₀) of up to 8% at 700 nm is introduced between the image pickupdevice in the rear of the zoom lens system and the object side of thesystem, the transmittance at a near-infrared area of 700 nm or longer isrelatively lower and the transmittance on the red side is relativelyhigher as compared with those of the absorption type, so that thetendency of bluish purple to turn into magenta—a defect of a CCD orother solid-state image pickup device having a complementary colorsfilter—is diminished by gain control and there can be obtained colorreproduction comparable to that by a CCD or other solid-state imagepickup device having a primary colors filter. In addition, it ispossible to improve on color reproduction of, to say nothing of primarycolors and complementary colors, objects having strong reflectivity inthe near-infrared range, like plants or the human skin.

Thus, it is preferable to satisfy the following conditions (22) and(23):

τ₇₀₀/τ₅₅₀≧0.8  (22)

τ₇₀₀/τ₅₅₀≦0.08  (23)

where τ₅₅₀ is the transmittance at 550 nm wavelength.

More preferably, the following conditions (22)′ and/or (23)′ should besatisfied:

τ₆₀₀/τ₅₅₀≧0.85  (22)

τ₇₀₀/τ₅₅₀≦0.05  (23)

Even more preferably, the following conditions (22)″ or (23)″ should besatisfied:

τ₆₀₀/τ₅₅₀≧0.9  (22)″

τ₇₀₀/τ₅₅₀≦0.03  (23)″

Most preferably, both conditions (28)″ and (29)′ should be satisfied.

Another defect of the CCD or other solid-state image pickup device isthat the sensitivity to the wavelength of 550 nm in the near ultravioletrange is considerably higher than that of the human eye. This, too,makes noticeable chromatic blurring at the edges of an image due tochromatic aberrations in the near-ultraviolet range. Such color blurringis fatal to a compact optical system. Accordingly, if an absorber orreflector is inserted on the optical path, which is designed such thatthe ratio of the transmittance (τ₄₀₀) at 400 nm wavelength to that(τ₅₅₀) at 550 nm wavelength is less than 0.08 and the ratio of thetransmittance (τ₄₄₀) at 440 nm wavelength to that (τ₅₅₀) at 550 nmwavelength is greater than 0.4, it is then possible to considerablyreduce noises such as chromatic blurring while the wavelength rangenecessary for color reproduction (satisfactory color reproduction) iskept intact.

It is thus preferable to satisfy the following conditions (24) and (25):

τ₄₀₀/τ₅₅₀≦0.08  (24)

τ₄₄₀/τ₅₅₀≧0.4  (25)

More preferably, the following conditions (24)′ and/or (25)′ should besatisfied.

τ₄₀₀/τ₅₅₀≦0.06  (24)′

τ₄₄₀/τ₅₅₀≧0.5  (25)′

Even more preferably, the following condition (24)″ or (25)″ should besatisfied.

τ₄₀₀/τ₅₅₀≦0.04  (24)″

τ₄₄₀/τ₅₅₀≧0.6  (25)″

Most preferably, both condition (24)″ and (25)″ should be satisfied.

It is noted that these filters should preferably be located between theimage-formation optical system and the image pickup device.

On the other hand, a complementary colors filter is higher insubstantial sensitivity and more favorable in resolution than a primarycolors filter-inserted CCD due to its high transmitted light energy, andprovides a great merit when used in combination with a small-size CCD.

To shorten and slim down the optical system, the optical low-pass filterthat is another filter, too, should preferably be thinned as much aspossible. In general, an optical low-pass filter harnesses adouble-refraction action that a uniaxial crystal like berg crystal has.However, when the optical low-pass filter includes a quartz opticallow-pass filter or filters in which the angles of the crystal axes withrespect to the optical axis of the zoom lens are in the range of 35° to55° and the crystal axes are in varying directions upon projected ontothe image plane, the filter having the largest thickness along theoptical axis of the zoom lens among them should preferably satisfy thefollowing condition (26) with respect to its thickness t_(LPF) (mm).

0.08<t _(LPF) /a<0.16 (at a<4 μm)

0.075<t _(LPF) /a<0.15 (at a<3 μm)  (26)

Here t_(LPF) (mm) is the thickness of the optical low-pass filter havingthe largest thickness along the optical axis of the zoom lens with theangle of one crystal axis with respect to the optical axis being in therange of 35° to 55°, and a is the horizontal pixel pitch (in μm) of theimage pickup device.

Referring to a certain optical low-pass filter or an optical low-passfilter having the largest thickness among optical low-pass filters, itsthickness is set in such a way that contrast becomes theoretically zeroat the Nyquist threshold wavelength, i.e., at approximately a/5.88 (mm).A thicker optical low-pass filter may be effective for prevention ofswindle signals such as moire fringes, but makes it impossible to takefull advantages of the resolving power that the electronic image pickupdevice has, while a thinner filter renders full removal of swindlesignals like moire fringes impossible. However, swindle signals likemoire fringes have close correlations with the image-formationcapability of a taking lens like a zoom lens; high image-formationcapability renders swindle signals like moire fringes likely to occur.Accordingly, when the image-formation capability is high, the opticallow-pass filter should preferably be somewhat thicker whereas when it islow, the optical low-pass filter should preferably be somewhat thinner.

As the pixel pitch becomes small, on the other hand, the contrast offrequency components greater than the Nyquist threshold decreases due tothe influence of diffraction by the image-formation lens system and,hence, swindle signals like moire fringes are reduced. Thus, it ispreferable to reduce the thickness of the optical low-pass filter by afew % or a few tens % from a/5.88 (mm) because a rather improvedcontrast is obtainable at a spatial frequency lower than the frequencycorresponding to the Nyquist threshold.

More preferably,

0.075<t _(LPF) /a<0.15 (at a<4 μm)

0.07<t _(LPF) /a<0.14 (at a<3 μm)  (26)′

Most preferably,

0.07<t _(LPF) /a<0.14 (at a<4 μm)

0.065<t _(LPF) /a<0.13 (at a<3 μm)  26)″

If an optical low-pass filter is too thin at a<4 μm, it is thendifficult to process. Thus, it is permissible to impart some thicknessto the optical low-pass filter or make high the spatial frequency(cutoff frequency) where contrast reduces down to zero even when theupper limit to conditions (26), (26)′ and (26)″ is exceeded. In otherwords, it is permissible to regulate the angle of the crystal axis ofthe optical low-pass filter with respect to the optical axis of the zoomlens to within the range of 15° to 35° or 55° to 75°. In some cases, itis also permissible to dispense with the optical low-pass filter. Inthat angle range, the quantity of separation of incident light to anordinary ray and an extraordinary ray is smaller than that around 45°,and that separation does not occur at 0° or 90° (at 90°, however, thereis a phase difference because of a velocity difference between bothrays—the quarter-wave principle).

As already described, when the pixel pitch becomes small, it isdifficult to increase the F-number because the image-formationcapability deteriorates under the influence of diffraction at a highspatial frequency that compensates for such a small pixel pitch. It isthus acceptable to use two types of aperture stops for a camera, i.e., afull-aperture stop where there is a considerable deterioration due togeometric aberrations and an aperture stop having an F-number in thevicinity of diffraction limited. It is then acceptable to dispense withsuch an optical low-pass filter as described before.

Especially when the pixel pitch is small and the highest image-formationcapability is obtained at a full-aperture stop, etc., it is acceptableto use an aperture stop having a constantly fixed inside diameter asmeans for controlling the size of an incident light beam on the imagepickup plane instead of using an aperture stop having a variable insidediameter or a replaceable aperture stop. Preferably in that case, atleast one of lens surfaces adjacent to the aperture stop should be setsuch that its convex surface is directed to the aperture stop and itextends through the inside diameter portion of the aperture stop,because there is no need of providing any additional space for the stop,contributing to length reductions of the zoom optical system. It is alsodesirable to locate an optical element having a transmittance of up to90% (where possible, the entrance and exit surfaces of the opticalelement should be defined by planar surfaces) in a space including theoptical axis at least one lens away from the aperture stop or use meansfor replacing that optical element by another element having a differenttransmittance.

Alternatively, the electronic imaging system is designed in such a wayas to have a plurality of apertures each of fixed aperture size, one ofwhich can be inserted into any one of optical paths between the lenssurface located nearest to the image side of the first lens group andthe lens surface located nearest to the object side of the third lensgroup and can be replaced with another as well, so that illuminance onthe image plane can be adjusted. Then, media whose transmittances withrespect to 550 nm are different but less than 80% are filled in some ofthe plurality of apertures for light quantity control. Alternatively,when control is carried out in such a way as to provide a light quantitycorresponding to such an F-number as given by a (μm)/F-number<4.0, it ispreferable to fill the apertures with medium whose transmittance withrespect to 550 nm are different but less than 80%. In the range of thefull-aperture value to values deviating from the aforesaid condition asan example, any medium is not used or dummy media having a transmittanceof at least 91% with respect to 550 nm are used. In the range of theaforesaid condition, it is preferable to control the quantity of lightwith an ND filter or the like, rather than to decrease the diameter ofthe aperture stop to such an extent that the influence of diffractionappears.

Alternatively, it is acceptable to uniformly reduce the diameters of aplurality of apertures inversely with the F-numbers, so that opticallow-pass filters having different frequency characteristics can beinserted in place of ND filters. As degradation by diffraction becomesworse with stop-down, it is desirable that the smaller the aperturediameter, the higher the frequency characteristics the optical low-passfilters have.

It is understood that when the relation of the full-aperture F-number atthe wide-angle end to the pixel pitch a (μm) used satisfies F>a, it isacceptable to dispense with the optical low-pass filter. In other words,it is permissible that the all the medium on the optical axis betweenthe zoom lens system and the electronic image pickup device is composedof air or a non-crystalline medium alone. This is because there arelittle frequency components capable of producing distortions uponbending due to a deterioration in the image-formation capability byreason of diffraction and geometric aberrations.

It is noted that satisfactory zoom lenses or electronic imaging systemsmay be set up by suitable combinations of the above conditions and thearrangements of the zoom lens and the electronic imaging system usingthe same.

It is understood that only the upper limit or only the lower limit maybe applied to each of the above conditions, and that the values of theseconditions in each of the following examples may be extended as far asthe upper or lower limits thereof.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts that will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), 1(b) and 1(c) are illustrative in section of Example 1 ofthe zoom lens according to the present invention at the wide-angle end(a), in an intermediate state (b) and at the telephoto end (c),respectively, when the zoom lens is focused on an object point atinfinity.

FIGS. 2(a), 2(b) and 2(c) are illustrative in section of Example 2 ofthe zoom lens, similar to FIGS. 1(a) to 1(c).

FIGS. 3(a), 3(b) and 3(c) are sections in schematic illustrative ofExample 3 of the zoom lens, similar to FIGS. 1(a) to 1(c).

FIGS. 4(a), 4(b) and 4(c) are illustrative in section of Example 4 ofthe zoom lens, similar to FIGS. 1(a) to 1(c).

FIGS. 5(a), 5(b) and 5(c) are illustrative in section of Example 5 ofthe zoom lens, similar to FIGS. 1(a) to 1(c).

FIG. 6 is an optical path diagram for Example 1 of the zoom lens whenthe optical path is bent upon focused on an infinite object point at thewide-angle end.

FIG. 7 is illustrative of the diagonal length of the effective imagepickup plane of an electronic image pickup device upon phototaking.

FIG. 8 is a diagram indicative of the transmittance characteristics ofone example of the near-infrared sharp cut coat.

FIG. 9 is a diagram indicative of the transmittance characteristics ofone example of the color filter located on the exit surface side of thelow-pass filter.

FIG. 10 is a schematic illustrative of how the color filter elements arearranged in the complementary colors mosaic filter.

FIG. 11 is a diagram indicative of one example of the wavelengthcharacteristics of the complementary colors mosaic filter.

FIG. 12 is a perspective view of details of one example of an aperturestop portion used in each example.

FIGS. 13(a) and 13(b) are illustrative in detail of another example ofthe aperture stop portion used in each example.

FIG. 14 is a front perspective schematic illustrative of the outsideshape of a digital camera in which the optical path-bending zoom opticalsystem of the present invention is built.

FIG. 15 is a rear perspective schematic of the digital camera of FIG.14.

FIG. 16 is a sectional schematic of the digital camera of FIG. 14.

FIG. 17 is a front perspective view of an uncovered personal computer inwhich the optical path-bending zoom optical system of the presentinvention is built as an objective optical system.

FIG. 18 is a sectional view of a phototaking optical system for apersonal computer.

FIG. 19 is a side view of the state of FIG. 17.

FIGS. 20(a) and 20(b) are a front and a side view of cellular phone inwhich the optical path-bending zoom optical system of the presentinvention is built as an objective optical system, and FIG. 20(c) is asectional view of a phototaking optical system for the same.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples 1 to 5 of the zoom lens according to the present invention arenow explained. Sectional lens configurations of Examples 1 to 5 at thewide-angle end (a), in the intermediate state (b) and at the telephotoend (c) upon focused on an object point at infinity are shown in FIGS. 1to 5. Throughout FIGS. 1 to 5, the first lens group is indicated by G1,the second lens group by G2, a stop by S, the third lens group by G3,the fourth lens group by G4, an optical low-pass filter by LF, a coverglass for an electronic image pickup device CCD by CG, and the imageplane of CCD by I. A plane-parallel plate or the taken-apart opticalpath-bending prism in the first lens group G1 is indicated by P. Themaximum thickness of the optical low-pass filter LF used in theseexamples will be explained later. It is noted that instead of thenear-infrared sharp cut coat, it is acceptable to use an opticallow-pass filter LF coated directly with a near-infrared sharp cut coat,an infrared cut absorption filter or a transparent plane plate with anear-infrared sharp cut coat applied on its entrance surface.

As shown typically in FIG. 6 that is an optical path diagram for Example1 of the zoom lens upon focused on an infinite object point at thewide-angle end, the optical path-bending prism P is configured as areflecting prism for bending the optical path through 90°.

EXAMPLE 1

As shown in FIGS. 1(a), 1(b) and 1(c), Example 1 is directed to a zoomlens made up of a first lens group G1 composed of a negative meniscuslens element convex on its object side, an optical path-bending prism Pand a double-convex positive lens element, a second lens group G2composed of a double-concave negative lens element and a positivemeniscus lens element convex on its object side, an aperture stop S, athird lens group G3 composed of a doublet consisting of a double-convexpositive lens element and a double-concave lens element and a fourthlens group G4 composed of one positive meniscus lens element convex onits object side. Upon the wide-angle end to the telephoto end of thezoom lens, the first lens group G1 and the aperture stop S remain fixed,the second lens group G2 moves toward the image plane side of the zoomlens, the third lens group G3 moves toward the object side of the zoomlens, and the fourth lens group G4 moves toward the image plane side.For focusing on a nearby subject, the fourth lens group G4 moves towardthe object side.

Five aspheric surfaces are used; two at both surfaces of thedouble-concave negative lens element in the second lens group G2, two atthe surfaces nearest to the object and image plane sides of the thirdlens group G3 and one at the object side-surface of the positivemeniscus lens element in the fourth lens group G4.

EXAMPLE 2

As shown in FIGS. 2(a), 2(b) and 2(c), Example 2 is directed to a zoomlens made up of a first lens group G1 composed of a negative meniscuslens element convex on its object side, an optical path-bending prism Pand a double-convex positive lens element, a second lens group G2composed of a double-concave negative lens element and a positivemeniscus lens element convex on its object side, an aperture stop S, athird lens group G3 composed of a doublet consisting of a double-convexpositive lens element and a double-concave negative lens element and afourth lens group G4 composed of one positive meniscus lens elementconvex on its object side. Upon the wide-angle end to the telephoto endof the zoom lens, the first lens group G1 and the aperture stop S remainfixed, the second lens group G2 moves toward the image plane side of thezoom lens, the third lens group G3 moves toward the object side of thezoom lens, and the fourth lens group G4 moves toward the image planeside. For focusing on a nearby subject, the fourth lens group G4 movestoward the object side.

Four aspheric surfaces are used; one at the image plane side-surface ofthe double-concave negative lens element in the second lens group G2,two at both surfaces of the double-convex positive lens element on theobject side of the third lens group G3 and one at the objectside-surface of the positive meniscus lens element in the fourth lensgroup G4.

EXAMPLE 3

As shown in FIGS. 3(a), 3(b) and 3(c), Example 3 is directed to a zoomlens made up of a first lens group G1 composed of a negative meniscuslens element on its object side, an optical path-bending prism P and adouble-convex positive lens element, a second lens group G2 composed ofa double-concave negative lens element and a positive meniscus lenselement convex on its object side, an aperture stop S, a third lensgroup G3 composed of a double-convex positive lens element and a doubletconsisting of a double-convex positive lens element and a double-concavenegative lens element, and a fourth lens group G4 composed of onepositive meniscus lens element convex on its object side. Upon thewide-angle end to the telephoto end of the zoom lens, the first lensgroup G1 and the aperture stop S remain fixed, the second lens group G2moves toward the image plane side of the zoom lens, the third lens groupG3 moves toward the object side of the zoom lens, and the fourth lensgroup G4 moves toward the image plane side. For focusing on a nearbysubject, the fourth lens group G4 moves toward the object side.

Four aspheric surfaces are used; on at the image plane side-surface ofthe double-concave negative lens element in the second lens group G2,two at both surfaces of the double-convex positive lens element on theobject side of the third lens group G3 and one at the objectside-surface of the positive meniscus lens element in the fourth lensgroup G4.

EXAMPLE 4

As shown in FIGS. 4(a), 4(b) and 4(c), Example 4 is directed to a zoomlens made up of a first lens group G1 composed of a negative meniscuslens element convex on its object side, an optical path-bending prism Pand a double-convex positive lens element, a second lens group G2composed of a double-concave negative lens element and a double-convexpositive lens element, an aperture stop S, a third lens group G3composed of a doublet consisting of a double-convex positive lenselement and a double-concave negative lens element and a meniscus lenselement convex on its object side and a fourth lens group G4 composed ofone positive meniscus lens element convex on its object side. Upon thewide-angle end to the telephoto end of the zoom lens, the first lensgroup G1 and the aperture stop S remain fixed, the second lens group G2moves toward the image plane side of the zoom lens, the third lens groupG3 moves toward the object side of the zoom lens, and the fourth lensgroup G4 moves slightly toward the object side and then toward the imageplane side. For focusing on a nearby subject, the fourth lens group G4moves toward the object side.

Five aspheric surfaces are used; two at both surfaces of thedouble-concave negative lens element in the second lens group G2, one atthe object side-surface of the doublet in the third lens group G3 andtwo at both surface of the meniscus lens element in the third lens groupG3.

EXAMPLE 5

As shown in FIGS. 5(a), 5(b) and 5(c), Example 5 is directed to a zoomlens made up of a first lens group G1 composed of a negative meniscuslens element convex on its object side, an optical path-bending prism Pand a double-convex positive lens element, a second lens group G2composed of a doublet consisting of a double-concave negative lenselement and a negative meniscus lens element convex on its object side,an aperture stop S, a third lens group G3 composed of a double-convexpositive lens element and a doublet consisting of a positive meniscuslens element convex on its object side and a negative meniscus lenselement convex on its object side and a fourth lens group G4 composed ofone positive meniscus lens element convex on its object side. Upon thewide-angle end to the telephoto end of the zoom lens, the first lensgroup G1 and the aperture stop S remain fixed, the second lens group G2moves toward the image plane side of the zoom lens, the third lens groupG3 moves toward the object side of the zoom lens, and the fourth lensgroup G4 moves toward the image plane side. For focusing on a nearbysubject, the fourth lens group G4 moves toward the object side.

Four aspheric surfaces are used; one at the image plane side-surface ofthe negative meniscus lens element in the first lens group G1, two atboth surfaces of the double-convex positive lens element in the thirdlens group G3 and one at the object side-surface of the positivemeniscus lens element in the fourth lens group G4.

The numerical data on each example are given below. Symbols usedhereinafter but not hereinbefore have the following meanings:

f: focal length of the zoom lens

F_(NO): F-number

ω: half angle of view

WE: wide-angle end

ST: intermediate state

TE: telephoto end

r₁, r₂, . . . : radius of curvature of each lens surface

d₁, d₂, . . . : spacing between the adjacent lens surfaces

n_(d1), n_(d2), . . . : d-line refractive index of each lens element

V_(d1), V_(d2), . . . : Abbe number of each lens element

Here let x be an optical axis on condition that the direction ofpropagation of light is positive and y be a direction perpendicular tothe optical axis. Then, aspheric configuration is given by

x=(y ² /r)/[1+{1−(K+1)(y/r)²}^(1/2) ]+A ₄ y ⁴ +A ₆ y ⁶ +A ₈ y ⁸ +A ₁₀ y¹⁰

where r is a paraxial radius of curvature, K is a conical coefficient,and A₄, A₆, A₈ and A₁₀ are the fourth, sixth, eighth and tenth asphericcoefficients, respectively.

EXAMPLE 1

r₁ = 31.0100 d₁ = 1.0000 n_(d1) = 1.80100 ν_(d1) = 34.97 r₂ = 9.9641 d₂= 2.9000 r₃ = ∞ d₃ = 12.0000 n_(d2) = 1.80610 ν_(d2) = 40.92 r₄ = ∞ d₄ =0.3000 r₅ = 23.6950 d₅ = 3.5400 n_(d3) = 1.74100 ν_(d3) = 52.64 r₆ =−23.6475 d₆ = (Variable) r₇ = −377.9014 (Aspheric) d₇ = 0.8000 n_(d4) =1.80610 ν_(d4) = 40.92 r₈ = 6.4536 (Aspheric) d₈ = 0.7000 r₉ = 6.8913 d₉= 2.2000 n_(d5) = 1.75520 ν_(d5) = 27.51 r₁₀ = 16.1043 d₁₀ = (Variable)r₁₁ = ∞ (Stop) d₁₁ = (Variable) r₁₂ = 7.5543 (Aspheric) d₁₂ = 6.1695n_(d6) = 1.74320 ν_(d6) = 49.34 r₁₃ = −13.0000 d₁₃ = 1.0000 n_(d7) =1.84666 ν_(d7) = 23.78 r₁₄ = 13.1848 (Aspheric) d₁₄ = (Variable) r₁₅ =12.3030 (Aspheric) d₁₅ = 1.8000 n_(d8) = 1.74320 ν_(d8) = 49.34 r₁₆ =1061.3553 d₁₆ = (Variable) r₁₇ = ∞ d₁₇ = 1.9000 n_(d9) = 1.54771 ν_(d9)= 62.84 r₁₈ = ∞ d₁₈ = 0.8000 r₁₉ = ∞ d₁₉ = 0.7500 n_(d10) = 1.51633ν_(d10) = 64.14 r₂₀ = ∞ d₂₀ = 1.3565 r₂₁ = ∞ (Image Plane) AsphericalCoefficients 7th surface K = 0 A₄ = 5.2999 × 10⁻⁴ A₆ = −2.1607 × 10⁻⁵ A₈= 1.8300 × 10⁻⁷ A₁₀ = 0.0000 8th surface K = 0 A₄ = 5.8050 × 10⁻⁴ A₆ =−1.0603 × 10⁻⁵ A₈ = −7.5526 × 10⁻⁷ A₁₀ = 0.0000 12th surface K = 0 A₄ =5.1734 × 10⁻⁵ A₆ = 1.0455 × 10⁻⁶ A₈ = −3.4185 × 10⁻⁸ A₁₀ = 0.0000 14thsurface K = 0 A₄ = 8.4429 × 10⁻⁴ A₆ = 2.1473 × 10⁻⁵ A₈ = 7.3738 × 10⁻⁷A₁₀ = 0.0000 15th surface K = 0 A₄ = −6.2738 × 10⁻⁵ A₆ = 7.6642 × 10⁻⁶A₈ = −2.0106 × 10⁻⁷ A₁₀ = 0.0000 Zooming Data (∞) WE ST TE f (mm)6.01125 10.40282 17.99133 F_(NO) 2.5820 3.5145 4.7679 ω (°) 32.7 19.611.4 d₆ 0.78801 4.80346 8.70695 d₁₀ 9.39271 5.38074 1.47422 d₁₁ 11.133205.78312 1.48451 d₁₄ 2.19671 8.56256 14.78227 d₁₆ 4.12457 3.11055 1.18821

r₁ = 31.1674 d₁ = 1.0000 n_(d1) = 1.80518 ν_(d1) = 25.42 r₂ = 10.0082 d₂= 2.8000 r₃ = ∞ d₃ = 12.0000 n_(d2) = 1.80610 ν_(d2) = 40.92 r₄ = ∞ d₄ =0.3000 r₅ = 38.3752 d₅ = 3.3000 n_(d3) = 1.77250 ν_(d3) = 49.60 r₆ =−19.0539 d₆ = (Variable) r₇ = −27.7782 d₇ = 1.0000 n_(d4) = 1.80610ν_(d4) = 40.92 r₈ = 5.9968 (Aspheric) d₈ = 0.7000 r₉ = 8.0742 d₉ =2.3000 n_(d5) = 1.75520 ν_(d5) = 27.51 r₁₀ = −358.1053 d₁₀ = (Variable)r₁₁ = ∞ (Stop) d₁₁ = (Variable) r₁₂ = 8.4600 (Aspheric) d₁₂ = 2.5000n_(d6) = 1.74320 ν_(d6) = 49.34 r₁₃ = −116.7590 (Aspheric) d₁₃ = 0.1500r₁₄ = 8.8060 d₁₄ = 3.0000 n_(d7) = 1.60311 ν_(d7) = 60.64 r₁₅ = −40.0000d₁₅ = 0.7000 n_(d8) = 1.84666 ν_(d8) = 23.78 r₁₆ = 4.6054 d₁₆ =(Variable) r₁₇ = 6.7337 (Aspheric) d₁₇ = 1.9700 n_(d9) = 1.69350 ν_(d9)= 53.21 r₁₈ = 14.1820 d₁₈ = (Variable) r₁₉ = ∞ d₁₉ = 1.9000 n_(d10) =1.54771 ν_(d10) = 62.84 r₂₀ = ∞ d₂₀ = 0.8000 r₂₁ = ∞ d₂₁ = 0.7500n_(d11) = 1.51633 ν_(d11) = 64.14 r₂₂ = ∞ d₂₂ = 1.3596 r₂₃ = ∞ (ImagePlane) Aspherical Coefficients 8th surface K = 0 A₄ = −2.7926 × 10⁻⁴ A₆= −5.5281 × 10⁻⁶ A₈ = −3.0031 × 10⁻⁷ A₁₀ = 0.0000 12th surface K = 0 A₄= −1.0549 × 10⁻⁴ A₆ = −1.1474 × 10⁻⁶ A₈ = −5.2653 × 10⁻⁸ A₁₀ = 0.000013th surface K = 0 A₄ = −4.5663 × 10⁻⁵ A₆ = 6.3255 × 10⁻⁶ A₈ = −3.7416 ×10⁻⁷ A₁₀ = 0.0000 17th surface K = 0 A₄ = −3.4690 × 10⁻⁴ A₆ = 2.1996 ×10⁻⁶ A₈ = −1.8422 × 10⁻⁷ A₁₀ = 0.0000 Zooming Data (∞) WE ST TE f (mm)6.00633 10.39946 17.99885 F_(NO) 2.8069 3.3441 4.0747 ω (°) 32.4 18.910.9 d₆ 0.79862 7.41546 13.08585 d₁₀ 13.68612 7.06296 1.39894 d₁₁7.73864 4.51502 1.19986 d₁₆ 1.69904 5.23999 10.27759 d₁₈ 3.54003 3.222461.50021

r₁ = 31.4475 d₁ = 1.0000 n_(d1) = 1.80518 ν_(d1) = 25.42 r₂ = 10.0029 d₂= 2.8000 r₃ = ∞ d₃ = 12.0000 n_(d2) = 1.80610 ν_(d2) = 40.92 r₄ = ∞ d₄ =0.3000 r₅ = 40.9109 d₅ = 3.1000 n_(d3) = 1.77250 ν_(d3) = 49.60 r₆ =−18.5523 d₆ = (Variable) r₇ = −27.7365 d₇ = 0.9000 n_(d4) = 1.80610ν_(d4) = 40.92 r₈ = 6.1675 (Aspheric) d₈ = 0.6000 r₉ = 7.8689 d₉ =2.5000 n_(d5) = 1.75520 ν_(d5) = 27.51 r₁₀ = 541.9130 d₁₀ = (Variable)r₁₁ = ∞ (Stop) d₁₁ = (Variable) r₁₂ = 6.8303 (Aspheric) d₁₂ = 2.2000n_(d6) = 1.74320 ν_(d6) = 49.34 r₁₃ = −168.3254 (Aspheric) d₁₃ = 0.1500r₁₄ = 10.3767 d₁₄ = 2.5000 n_(d7) = 1.60311 ν_(d7) = 60.64 r₁₅ =−100.0000 d₁₅ = 0.7000 n_(d8) = 1.84666 ν_(d8) = 23.78 r₁₆ = 4.2552 d₁₆= (Variable) r₁₇ = 6.4363 (Aspheric) d₁₇ = 2.0000 n_(d9) = 1.58313ν_(d9) = 59.38 r₁₈ = 16.8235 d₁₈ = (Variable) r₁₉ = ∞ d₁₉ = 1.5000n_(d10) = 1.54771 ν_(d10) = 62.84 r₂₀ = ∞ d₂₀ = 0.8000 r₂₁ = ∞ d₂₁ =0.7500 n_(d11) = 1.51633 ν_(d11) = 64.14 r₂₂ = ∞ d₂₂ = 1.3596 r₂₃ = ∞(Image Plane) Aspherical Coefficients 8th surface K = 0 A₄ = −2.1223 ×10⁻⁴ A₆ = −3.9476 × 10⁻⁶ A₈ = −2.3492 × 10⁻⁷ A₁₀ = 0.0000 12th surface K= 0 A₄ = −9.9966 × 10⁻⁵ A₆ = −4.8770 × 10⁻⁶ A₈ = 7.8835 × 10⁻⁷ A₁₀ =0.0000 13th surface K = 0 A₄ = 1.6853 × 10⁻⁴ A₆ = 4.2908 × 10⁻⁶ A₈ =8.3613 × 10⁻⁷ A₁₀ = 0.0000 17th surface K = 0 A₄ = −3.5205 × 10⁻⁴ A₆ =−1.4117 × 10⁻⁶ A₈ = −1.1635 × 10⁻⁷ A₁₀ = 0.0000 Zooming Data (∞) WE STTE f (mm) 6.00728 10.39935 17.99830 F_(NO) 2.7463 3.3017 4.0273 ω (°)32.4 18.9 11.0 d₆ 0.79769 7.29414 13.01239 d₁₀ 13.61214 7.11013 1.39751d₁₁ 7.70485 4.37777 1.19903 d₁₆ 1.69969 5.42936 10.44566 d₁₈ 3.740843.33843 1.50064

r₁ = 32.0016 d₁ = 1.0000 n_(d1) = 1.75520 ν_(d1) = 27.51 r₂ = 10.0102 d₂= 2.8000 r₃ = ∞ d₃ = 12.0000 n_(d2) = 1.80610 ν_(d2) = 40.92 r₄ = ∞ d₄ =0.3000 r₅ = 23.5519 d₅ = 3.1000 n_(d3) = 1.72916 ν_(d3) = 54.68 r₆ =−24.7555 d₆ = (Variable) r₇ = −21.9861 (Aspheric) d₇ = 0.9000 n_(d4) =1.80610 ν_(d4) = 40.92 r₈ = 5.7215 (Aspheric) d₈ = 0.6000 r₉ = 7.9386 d₉= 2.5000 n_(d5) = 1.78470 ν_(d5) = 26.29 r₁₀ = −388.5176 d₁₀ =(Variable) r₁₁ = ∞ (Stop) d₁₁ = (Variable) r₁₂ = 5.6674 (Aspheric) d₁₂ =4.0000 n_(d6) = 1.74320 ν_(d6) = 49.34 r₁₃ = −19.0000 d₁₃ = 0.7000n_(d7) = 1.84666 ν_(d7) = 23.78 r₁₄ = 7.7986 d₁₄ = 0.3000 r₁₅ = 3.8662(Aspheric) d₁₅ = 1.0000 n_(d8) = 1.69350 ν_(d8) = 53.21 r₁₆ = 3.6817(Aspheric) d₁₆ = (Variable) r₁₇ = 13.0325 d₁₇ = 2.0000 n_(d9) = 1.48749ν_(d9) = 70.23 r₁₈ = 201.0398 d₁₈ = (Variable) r₁₉ = ∞ d₁₉ = 1.5000n_(d10) = 1.54771 ν_(d10) = 62.84 r₂₀ = ∞ d₂₀ = 0.8000 r₂₁ = ∞ d₂₁ =0.7500 n_(d11) = 1.51633 ν_(d11) = 64.14 r₂₂ = ∞ d₂₂ = 1.3599 r₂₃ = ∞(Image Plane) Aspherical Coefficients 7th surface K = 0 A₄ = 2.0496 ×10⁻⁴ A₆ = −3.4919 × 10⁻⁶ A₈ = 7.4208 × 10⁻⁹ A₁₀ = 0.0000 8th surface K =0 A₄ = −3.6883 × 10⁻⁴ A₆ = 3.4613 × 10⁻⁶ A₈ = −9.0209 × 10⁻⁷ A₁₀ =0.0000 12th surface K = 0 A₄ = 5.4882 × 10⁻⁴ A₆ = −1.8282 × 10⁻⁵ A₈ =1.6707 × 10⁻⁶ A₁₀ = 0.0000 15th surface K = 0 A₄ = −8.1049 × 10⁻³ A₆ =−4.3019 × 10⁻⁴ A₈ = −3.1973 × 10⁻⁵ A₁₀ = 0.0000 16th surface K = 0 A₄ =−6.4092 × 10⁻³ A₆ = −7.3362 × 10⁻⁴ A₈ = 2.9898 × 10⁻⁵ A₁₀ = 0.0000Zooming Data (∞) WE ST TE f (mm) 6.00844 10.40337 17.99810 F_(NO) 2.76592.9849 4.0444 ω (°) 32.6 19.2 11.3 d₆ 0.80018 8.47206 12.07930 d₁₀12.67757 5.00686 1.39837 d₁₁ 6.26991 5.19965 1.19782 d₁₆ 1.70036 2.603889.42234 d₁₈ 4.14771 4.30945 1.49796

r₁ = 37.5126 d₁ = 1.0000 n_(d1) = 1.78470 ν_(d1) = 26.29 r₂ = 9.9406(Aspheric) d₂ = 2.8000 r₃ = ∞ d₃ = 12.0000 n_(d2) = 1.80610 ν_(d2) =40.92 r₄ = ∞ d₄ = 0.3000 r₅ = 33.8530 d₅ = 3.1000 n_(d3) = 1.77250ν_(d3) = 49.60 r₆ = −21.7247 d₆ = (Variable) r₇ = −22.9665 d₇ = 0.9000n_(d4) = 1.77250 ν_(d4) = 49.60 r₈ = 7.9115 d₈ = 2.5000 n_(d5) = 1.71736ν_(d5) = 29.52 r₉ = 55.6404 d₉ = (Variable) r₁₀ = ∞ (Stop) d₁₀ =(Variable) r₁₁ = 8.1626 (Aspheric) d₁₁ = 2.2000 n_(d6) = 1.74320 ν_(d6)= 49.34 r₁₂ = −278.0091 (Aspheric) d₁₂ = 0.1500 r₁₃ = 7.0366 d₁₃ =2.5000 n_(d7) = 1.60311 ν_(d7) = 60.64 r₁₄ = 50.0000 d₁₄ = 0.7000 n_(d8)= 1.84666 ν_(d8) = 23.78 r₁₅ = 4.2115 d₁₅ = (Variable) r₁₆ = 6.7994(Aspheric) d₁₆ = 2.0000 n_(d9) = 1.58313 ν_(d9) = 59.38 r₁₇ = 13.6965d₁₇ = (Variable) r₁₈ = ∞ d₁₈ = 1.5000 n_(d10) = 1.54771 ν_(d10) = 62.84r₁₉ = ∞ d₁₉ = 0.8000 r₂₀ = ∞ d₂₀ = 0.7500 n_(d11) = 1.51633 ν_(d11) =64.14 r₂₁ = ∞ d₂₁ = 1.3586 r₂₂ = ∞ (Image Plane) Aspherical Coefficients2nd surface K = 0 A₄ = −4.8339 × 10⁻⁵ A₆ = 1.9771 × 10⁻⁷ A₈ = −1.3364 ×10⁻⁸ A₁₀ = 0.0000 11th surface K = 0 A₄ = −2.9041 × 10⁻⁴ A₆ = 2.3089 ×10⁻⁵ A₈ = −1.0828 × 10⁻⁶ A₁₀ = 0.0000 12th surface K = 0 A₄ = −1.9946 ×10⁻⁴ A₆ = 3.1348 × 10⁻⁵ A₈ = −1.4447 × 10⁻⁶ A₁₀ = 0.0000 16th surface K= 0 A₄ = −2.4256 × 10⁻⁴ A₆ = −6.3914 × 10⁻⁶ A₈ = 1.6763 × 10⁻⁷ A₁₀ =0.0000 Zooming Data (∞) WE ST TE f (mm) 6.02709 10.40552 17.99646 F_(NO)2.6193 3.3129 4.0433 ω (°) 32.3 18.9 11.0 d₆ 0.80042 6.82411 13.07966 d₉13.67313 7.63416 1.39413 d₁₀ 7.94928 4.18630 1.19879 d₁₅ 1.69392 6.1815710.44930 d₁₇ 3.50041 2.76626 1.49565

The values of conditions (1) to (25) in each example are enumeratedbelow with the values of t_(LPF) and L concerning condition (26). It isnoted that conditions (15) to (17) mean (15-1) to (15-3), (16-1) to(16-3) and (17-1) to (17-3), respectively.

Example 1 Example 2 Example 3 Example 4 Example 5 (1) 1.80053 1.798821.78926 1.89185 1.68172 (2) 1.58638 1.62590 1.62599 1.63599 1.68575 (3)1.34851 1.33482 1.33482 1.33482 1.33482 (4) 1.80610 1.80610 1.806101.80610 1.80610 (5) 0.91863 0.80674 0.81555 0.65256 0.69581 (6) 0.272290.29553 0.29058 0.35869 0.29828 (7) 0.94273 0.31220 0.32096 0.638120.74098 (8) 2.31092 2.42296 2.43781 2.46849 2.78836 (9) 1.62212 1.682251.69788 1.44993 1.75852 (10) 1.15319 1.17060 1.15739 1.13543 1.11669(11) 1.96930 1.50318 1.52111 1.28830 1.42870 (12) 1.21850 0.532160.53263 0.44969 0.54976 (13) 0.30433 0.31196 0.34434 0.52241 0.29698(14) 0.36543 0.28287 0.28291 0.28300 0.28105 (15) 1.74534 0.522980.41007 1.37605 0.59851 (16) −0.56154 −0.18250 −0.07300 −0.38421 0.14600(17) 25.56 36.86 36.86 25.56 36.86 (18) −1.02346 −2.80812 −2.23928−1.13863 −2.97167 (19) 0.43618 0.43762 0.43731 0.25625 0.34893 (20)0.00100 0.33644 0.37601 −0.02491 0.21822 (21) 0.96642 0.64490 0.636180.58701 0.48756 (22) 1.0 1.0 1.0 1.0 1.0 (23) 0.04 0.04 0.04 0.04 0.04(24) 0.0 0.0 0.0 0.0 0.0 (25) 1.06 1.06 1.06 1.06 1.06 a 3.5 3.9 3.7 2.92.5 t_(LPF) 0.55 0.58 0.52 0.38 0.30 L 7.30 7.30 7.30 7.30 7.30

Referring to the numerical data about Examples 1 to 5, it is understoodthat the optical low-pass filter is composed of a plurality of filterelements, and the thickness of the infrared cut filter, etc. is includedin such data. Thus, the maximum thickness corresponds to the value oft_(LPF) in the above table, rather than the value of t_(LPF). It is alsounderstood that any of the following ten combinations of a and t_(LPF)may be used.

1 2 3 4 5 a 3.5 3.9 3.7 2.9 2.5 t_(LPF) 0.55 0.58 0.52 0.38 0.30 6 7 8 910 a 2.8 2.7 2.6 3.3 3.1 t_(LPF) 0.25 0.25 0.26 0.24 0.25

Here the diagonal length L of the effective image pickup plane of theelectronic image pickup device and the pixel spacing a are explained.FIG. 7 is illustrative of one exemplary pixel array for the electronicimage pickup device, wherein R (red), G (green) and B (blue) pixels orfour pixels, i.e., cyan, magenta, yellow and green (G) pixels (see FIG.10) are mosaically arranged at the pixel spacing a. The “effective imagepickup plane” used herein is understood to mean a certain area in thephotoelectric conversion surface on an image pickup device used for thereproduction of a phototaken image (on a personal computer or by aprinter). The effective image pickup plane shown in FIG. 7 is set at anarea narrower than the total photoelectric conversion surface on theimage pickup device, depending on the performance of the optical systemused (an image circle that can be ensured by the performance of theoptical system). The diagonal length L of an effective image pickupplane is thus defined by that of the effective image pickup plane.Although the image pickup range used for image reproduction may bevariable, it is noted that when the zoom lens of the present inventionis used on an image pickup apparatus having such functions, the diagonallength L of its effective image pickup plane varies. In that case, thediagonal length L of the effective image pickup plane according to thepresent invention is defined by the maximum value in the widest possiblerange for L.

In each example of the present invention, on the image side of the finallens group there is provided a near-infrared cut filter or an opticallow-pass filter LF with a near-infrared cut coat surface applied on itsentrance side. This near-infrared cut filter or near-infrared cut coatsurface is designed to have a transmittance of at least 80% at 600 nmwavelength and a transmittance of up to 10% at 700 nm wavelength. Morespecifically, the near-infrared cut filter or the near-infrared sharpcut coat has a multilayer structure made up of such 27 layers asmentioned below; however, the design wavelength is 780 nm.

Substrate Material Physical Thickness (nm) λ/4 1st layer Al₂O₃ 58.960.50 2nd layer TiO₂ 84.19 1.00 3rd layer SiO₂ 134.14 1.00 4th layer TiO₂84.19 1.00 5th layer SiO₂ 134.14 1.00 6th layer TiO₂ 84.19 1.00 7thlayer SiO₂ 134.14 1.00 8th layer TiO₂ 84.19 1.00 9th layer SiO₂ 134.141.00 10th layer TiO₂ 84.19 1.00 11th layer SiO₂ 134.14 1.00 12th layerTiO₂ 84.19 1.00 13th layer SiO₂ 134.14 1.00 14th layer TiO₂ 84.19 1.0015th layer SiO₂ 178.41 1.33 16th layer TiO₂ 101.03 1.21 17th layer SiO₂167.67 1.25 18th layer TiO₂ 96.82 1.15 19th layer SiO₂ 147.55 1.05 20thlayer TiO₂ 84.19 1.00 21st layer SiO₂ 160.97 1.20 22nd layer TiO₂ 84.191.00 23rd layer SiO₂ 154.26 1.15 24th layer TiO₂ 95.13 1.13 25th layerSiO₂ 160.97 1.20 26th layer TiO₂ 99.34 1.18 27th layer SiO₂ 87.19 0.65

The aforesaid near-infrared sharp cut coat has such transmittancecharacteristics as shown in FIG. 8.

The low-pass filter LF is provided on its exit surface side with a colorfilter or coat for reducing the transmission of colors at such a shortwavelength region as shown in FIG. 9, thereby making the colorreproducibility of an electronic image much higher.

Preferably, that filter or coat should be designed such that the ratioof the transmittance of 420 nm wavelength with respect to thetransmittance of a wavelength in the range of 400 nm to 700 nm at whichthe highest transmittance is found is at least 15% and that the ratio of400 nm wavelength with respect to the highest wavelength transmittanceis up to 6%.

It is thus possible to reduce a discernible difference between thecolors perceived by the human eyes and the colors of the image to bepicked up and reproduced. In other words, it is possible to preventdegradation in images due to the fact that a color of short wavelengthless likely to be perceived through the human sense of sight can bereadily seen by the human eyes.

When the ratio of the 400 nm wavelength transmittance is greater than6%, the short wavelength region less likely to be perceived by the humaneyes would be reproduced with perceivable wavelengths. Conversely, whenthe ratio of the 420 nm wavelength transmittance is less than 15%, awavelength region perceivable by the human eyes is less likely to bereproduced, putting colors in an ill-balanced state.

Such means for limiting wavelengths can be more effective for imagingsystems using a complementary colors mosaic filter.

In each of the aforesaid examples, coating is applied in such a waythat, as shown in FIG. 9, the transmittance for 400 nm wavelength is 0%,the transmittance for 420 nm is 90%, and the transmittance for 440 nmpeaks or reaches 100%.

With the synergistic action of the aforesaid near-infrared sharp cutcoat and that coating, the transmittance for 400 nm is set at 0%, thetransmittance for 420 nm at 80%, the transmittance for 600 nm at 82%,and the transmittance for 700 nm at 2% with the transmittance for 450 nmwavelength peaking at 99%, thereby ensuring more faithful colorreproduction.

The low-pass filter LF is made up of three different filter elementsstacked one upon another in the optical axis direction, each filterelement having crystal axes in directions where, upon projected onto theimage plane, the azimuth angle is horizontal (=0°) and ±45° therefrom.Three such filter elements are mutually displaced by a μm in thehorizontal direction and by SQRT(½)×a in the ±45° direction for thepurpose of moiré control, wherein SQRT means a square root.

The image pickup plane I of a CCD is provided thereon with acomplementary colors mosaic filter wherein, as shown in FIG. 10, colorfilter elements of four colors, cyan, magenta, yellow and green arearranged in a mosaic fashion corresponding to image pickup pixels. Morespecifically, these four different color filter elements, used in almostequal numbers, are arranged in such a mosaic fashion that neighboringpixels do not correspond to the same type of color filter elements,thereby ensuring more faithful color reproduction.

To be more specific, the complementary colors mosaic filter is composedof at least four different color filter elements as shown in FIG. 10,which should preferably have such characteristics as given below.

Each green color filter element G has a spectral strength peak at awavelength G_(P),

each yellow filter element Y_(e) has a spectral strength peak at awavelength Y_(P),

each cyan filter element C has a spectral strength peak at a wavelengthC_(P), and

each magenta filter element M has spectral strength peaks at wavelengthsM_(P1) and M_(P2), and these wavelengths satisfy the followingconditions.

510 nm<G_(P)<540 nm

5 nm<Y _(P) −G _(P)<35 nm

 −100 nm<C _(P) −G _(P)<−5 nm

430 nm<M_(P1)<480 nm

580 nm<M_(P2)<640 nm

To ensure higher color reproducibility, it is preferred that the green,yellow and cyan filter elements have a strength of at least 80% at 530nm wavelength with respect to their respective spectral strength peaks,and the magenta filter elements have a strength of 10% to 50% at 530 nmwavelength with their spectral strength peak.

One example of the wavelength characteristics in the aforesaidrespective examples is shown in FIG. 11. The green filter element G hasa spectral strength peak at 525 nm. The yellow filter element Y_(e) hasa spectral strength peak at 555 nm. The cyan filter element C has aspectral strength peak at 510 nm. The magenta filter element M has peaksat 445 nm and 620 nm. At 530 nm, the respective color filter elementshave, with respect to their respective spectral strength peaks,strengths of 99% for G, 95% for Y_(e), 97% for C and 38% for M.

For such a complementary colors filter, such signal processing asmentioned below is electrically carried out by means of a controller(not shown) (or a controller used with digital cameras).

For luminance signals,

Y=|G+M+Y _(e) +C|×¼

For chromatic signals,

R−Y=|(M+Y _(e))−(G+C)|

 B−Y=|(M+C)−(G+Y _(e))|

Through this signal processing, the signals from the complementarycolors filter are converted into R (red), G (green) and B (blue)signals.

In this regard, it is noted that the aforesaid near-infrared sharp cutcoat may be located anywhere on the optical path, and that the number oflow-pass filters LF may be either two as mentioned above or one.

Details of the aperture stop portion in each example are shown in FIG.12 in conjunction with a four-group arrangement, wherein the first lensgroup G1 excepting the optical path-bending prism P is shown. At a stopposition on the optical axis between the first lens group G1 and thesecond lens group G2 in the phototaking optical system, there is locateda turret 10 capable of brightness control at 0 stage, −1 stage, −2stage, −3 stage and −4 stage. The turret 10 is composed of an aperture1A for 0 stage control, which is defined by a circular fixed space ofabout 4 mm in diameter (with a transmittance of 100% with respect to 550nm wavelength), an aperture 1B for −1 stage correction, which is definedby a transparent plane-parallel plate having a fixed aperture shape withan aperture area nearly half that of the aperture 1A (with atransmittance of 99% with respect to 550 nm wavelength), and circularapertures 1C, 1D and 1E for −2, −3 and −4 stage corrections, which havethe same aperture area as that of the aperture 1B and are provided withND filters having the respective transmittances of 50%, 25% and 13% withrespect to 550 nm wavelength.

By turning of the turret 10 around a rotating shaft 11, any one of theapertures is located at the stop position, thereby controlling thequantity of light.

The turret 10 is also designed that when the effective F-number F_(no)′is F_(no)′>a/0.4 μm, an ND filter with a transmittance of less than 80%with respect to 550 nm wavelength is inserted in the aperture. Referringspecifically to Example 1, the effective F-number at the telephoto endsatisfies the following condition when the effective F-number becomes9.0 at the −2 stage with respect to the stop-in (0) stage, and the thencorresponding aperture is 1C, whereby any image degradation due to adiffraction phenomenon by the stop is prevented.

Instead of the turret 10 shown in FIG. 12, it is acceptable to use aturret 10′ shown in FIG. 13(a). This turret 10′ capable of brightnesscontrol at 0 stage, −1 stage, −2 stage, −3 stage and −4 stage is locatedat the aperture stop position on the optical axis between the first lensgroup G1 and the second lens group G2 in the phototaking optical system.The turret 10′ is composed of an aperture 1A′ for 0 stage control, whichis defined by a circular fixed space of about 4 mm in diameter, anaperture 1B′ for −1 stage correction, which is of a fixed aperture shapewith an aperture area nearly half that of the aperture 1A′, andapertures 1C′, 1D′ and 1E′ for −2, −3 and −4 stage corrections, whichare of fixed shape with decreasing areas in this order. By turning ofthe turret 10′ around a rotating shaft 11, any one of the apertures islocated at the stop position thereby controlling the quantity of light.

Further, optical low-pass filters having varying spatial frequencycharacteristics are located in association with 1A′ to 1D′ of pluralsuch apertures. Then, as shown in FIG. 13(b), the spatial frequencycharacteristics of the optical filters are designed in such a way thatas the aperture diameter becomes small, they become high, therebypreventing image degradations due to a diffraction phenomenon bystop-down. Each curve in FIG. 13(b) is indicative of the spatialfrequency characteristics of the low-pass filters alone, wherein all thecharacteristics including diffraction by the stop are set in such a wayas to be equal to one another.

The present electronic imaging system constructed as described above maybe applied to phototaking systems where object images formed throughzoom lenses are received at image pickup devices such as CCDs orsilver-halide films, inter alia, digital cameras or video cameras aswell as PCs and telephone sets which are typical information processors,in particular, easy-to-carry cellular phones. Given below are some suchembodiments.

FIGS. 14, 15 and 16 are conceptual illustrations of a phototakingoptical system 41 for digital cameras, in which the zoom lens of thepresent invention is built. FIG. 14 is a front perspective view of theoutside shape of a digital camera 40, and FIG. 15 is a rear perspectiveview of the same. FIG. 16 is a horizontally sectional view of theconstruction of the digital camera 40. In this embodiment, the digitalcamera 40 comprises a phototaking optical system 41 including aphototaking optical path 42, a finder optical system 43 including afinder optical path 44, a shutter 45, a flash 46, a liquid crystalmonitor 47 and so on. As the shutter 45 mounted on the upper portion ofthe camera 40 is pressed down, phototaking takes place through thephototaking optical system 41, for instance, the optical path-bendingzoom lens according to Example 1. In this case, the optical path is bentby an optical path-bending prism P in the longitudinal direction of thedigital camera 40, i.e., in the lateral direction so that the camera canbe slimmed down. An object image formed by the phototaking opticalsystem 41 is formed on the image pickup plane of a CCD 49 via anear-infrared cut filter and an optical low-pass filter LF. The objectimage received at CCD 49 is shown as an electronic image on the liquidcrystal monitor 47 via processing means 51, which monitor is mounted onthe back of the camera. This processing means 51 is connected withrecording means 52 in which the phototaken electronic image may berecorded. It is here noted that the recording means 52 may be providedseparately from the processing means 51 or, alternatively, it may beconstructed in such a way that images are electronically recorded andwritten therein by means of floppy discs, memory cards, MOs or the like.This camera may also be constructed in the form of a silver halidecamera using a silver halide film in place of CCD 49.

Moreover, a finder objective optical system 53 is located on the finderoptical path 44. An object image formed by the finder objective opticalsystem 53 is in turn formed on the field frame 57 of a Porro prism 55that is an image erecting member. In the rear of the Porro prism 55there is located an eyepiece optical system 59 for guiding an erectedimage into the eyeball E of an observer. It is here noted that covermembers 50 are provided on the entrance sides of the phototaking opticalsystem 41 and finder objective optical system 53 as well as on the exitside of the eyepiece optical system 59.

With the thus constructed digital camera 40, it is possible to achievehigh performance and cost reductions, because the phototaking opticalsystem 41 is constructed of a fast zoom lens having a high zoom ratio atthe wide-angle end with satisfactory aberrations and a back focus largeenough to receive a filter, etc. therein. In addition, the camera can beslimmed down because, as described above, the optical path of the zoomlens is selectively bent in the longitudinal direction of the digitalcamera 40. With the optical path bent in the thus selected direction,the flash 46 is positioned above the entrance surface of the phototakingoptical system 42, so that the influences of shadows on strobe shots offigures can be slackened.

In the embodiment of FIG. 16, plane-parallel plates are used as thecover members 50; however, it is acceptable to use powered lenses. It isunderstood that depending on ease of camera's layout, the optical pathcan be bent in either one of the longitudinal and lateral directions.

FIGS. 17, 18 and 19 are illustrative of a personal computer that is oneexample of the information processor in which the image-formationoptical system of the present invention is built as an objective opticalsystem. FIG. 17 is a front perspective view of a personal computer 300that is in an uncovered state, FIG. 18 is a sectional view of aphototaking optical system 303 in the personal computer 300, and FIG. 19is a side view of the state of FIG. 17. As shown in FIGS. 17, 18 and 19,the personal computer 300 comprises a keyboard 301 via which an operatorenters information therein from outside, information processing orrecording means (not shown), a monitor 302 on which the information isshown for the operator, and a phototaking optical system 303 for takingan image of the operator and surrounding images. For the monitor 302,use may be made of a transmission type liquid crystal display deviceilluminated by backlight (not shown) from the back surface, a reflectiontype liquid crystal display device in which light from the front isreflected to show images, or a CRT display device. While the phototakingoptical system 303 is shown as being built in the right upper portion ofthe monitor 302, it may be located somewhere around the monitor 302 orkeyboard 301.

This phototaking optical system 303 comprises on a phototaking opticalpath 304 an objective lens 112 such as one represented by Example 1 ofthe optical path-bending zoom lens according to the present inventionand an image pickup device chip 162 for receiving an image.

Here an optical low-pass filter LF is additionally applied onto theimage pickup device chip 162 to form an integral imaging unit 160, whichcan be fitted into the rear end of a lens barrel 113 of the objectivelens 112 in one-touch operation. Thus, the assembly of the objectivelens 112 and image pickup device chip 162 is facilitated because of noneed of alignment or control of surface-to-surface spacing. The lensbarrel 113 is provided at its end (not shown) with a cover glass 114 forprotection of the objective lens 112. It is here noted that drivingmechanisms for the zoom lens, etc. contained in the lens barrel 113 arenot shown.

An object image received at the image pickup device chip 162 is enteredvia a terminal 166 in the processing means of the personal computer 300,and displayed as an electronic image on the monitor 302. As an example,an image 305 taken of the operator is shown in FIG. 17. This image 305may be displayed on a personal computer on the other end via suitableprocessing means and the Internet or telephone line.

FIGS. 20(a), 20(b) and 20(c) are illustrative of a telephone set that isone example of the information processor in which the image-formationoptical system of the present invention is built in the form of aphoto-taking optical system, especially a convenient-to-carry cellularphone. FIG. 20(a) and FIG. 20(b) are a front and a side view of acellular phone 400, respectively, and FIG. 34(c) is a sectional view ofa phototaking optical system 405. As shown in FIGS. 20(a), 20(b) and20(c), the cellular phone 400 comprises a microphone 401 for enteringthe voice of an operator therein as information, a speaker 402 forproducing the voice of the person on the other end, an input dial 403via which the operator enters information therein, a monitor 404 fordisplaying an image taken of the operator or the person on the other endand indicating information such as telephone numbers, a photo-takingoptical system 405, an antenna 406 for transmitting and receivingcommunication waves, and processing means (not shown) for processingimage information, communication information, input signals, etc. Herethe monitor 404 is a liquid crystal display device. It is noted that thecomponents are not necessarily arranged as shown. The phototakingoptical system 405 comprises on a phototaking optical path 407 anobjective lens 112 such as one represented by Example 1 of the opticalpath-bending zoom lens according to the present invention and an imagepickup device chip 162 for receiving an object image. These are built inthe cellular phone 400.

Here an optical low-pass filter LF is additionally applied onto theimage pickup device chip 162 to form an integral imaging unit 160, whichcan be fitted into the rear end of a lens barrel 113 of the objectivelens 112 in one-touch operation. Thus, the assembly of the objectivelens 112 and image pickup device chip 162 is facilitated because of noneed of alignment or control of surface-to-surface spacing. The lensbarrel 113 is provided at its end (not shown) with a cover glass 114 forprotection of the objective lens 112. It is here noted that drivingmechanisms for the zoom lens, etc. contained in the lens barrel 113 arenot shown.

An object image received at the image pickup device chip 162 is enteredvia a terminal 166 in processing means (not shown), so that the objectimage can be displayed as an electronic image on the monitor 404 and/ora monitor at the other end. The processing means also include a signalprocessing function for converting information about the object imagereceived at the image pickup device chip 162 into transmittable signals,thereby sending the image to the person at the other end.

The present invention provides a zoom lens that is well received at acollapsible lens mount with reduced thickness, has a high zoom ratio andshows excellent image-formation capability even upon rear-focusing. Withthis zoom lens, it is possible to thoroughly slim down video cameras ordigital cameras.

I claim:
 1. A zoom lens, comprising, in order from an object sidethereof, a first lens group that remains fixed during zooming, a secondlens group that has negative refracting power and moves during zooming,a third lens group that has positive refracting power and moves duringzooming, and a fourth lens group that has positive refracting power andmoves during zooming and focusing, wherein: the first lens groupcomprises, in order from an object side thereof, a negative meniscuslens component convex on an object side thereof, a reflecting opticalelement for bending an optical path and a positive lens.
 2. A zoom lens,comprising, in order from an object side thereof, a first lens groupthat remains fixed during zooming, a second lens group that has negativerefracting power and moves during zooming, a third lens group that haspositive refracting power and moves during zooming, and a fourth lensgroup that has positive refracting power and moves during zooming andfocusing, wherein: the first lens group comprises a reflecting opticalelement for bending an optical path, and upon focusing on an infiniteobject point, the fourth lens group moves in a locus opposite to that ofmovement of the third lens group during zooming.
 3. The zoom lensaccording to claim 1, wherein upon focusing, the fourth lens group movesin a locus opposite to that of movement of the third lens group duringzooming.
 4. The zoom lens according to claim 1, wherein upon focusing,only the forth lens group moves.
 5. The zoom lens according to claim 1,wherein the third lens group comprises a doublet component in which apositive lens and a negative lens are cemented together, and the thirdlens group comprises a lens component with both surfaces being definedby aspheric surfaces.
 6. The zoom lens according to claim 1, wherein thethird lens group comprises a doublet component in which a positive lensand a negative lens t are cemented together, and the third lens groupcomprises a positive lens element having an aspheric surface.
 7. Thezoom lens according to claim 1, wherein the third lens group comprises adoublet component in which a positive lens and a negative lens arecemented together, and the third lens group comprises two asphericsurfaces.
 8. The zoom lens according to claim 1, which satisfies thefollowing conditions (1) and (2): 1.4<−f ₁₁/{square root over ( )}(f_(W) ·f _(T))<2.4  (1) 1.2<f ₁₂/{square root over ( )}(f _(W) ·f_(T))<2.2  (2) where f₁₁ is a focal length of the negative meniscus lenscomponent in the first lens group, F₁₂ is a focal length of the positivelens in the first lens group, and f_(W) and f_(T) are focal lengths ofthe zoom lens at a wide-angle end and a telephoto end of the zoom lens,respectively.
 9. The zoom lens according to claim 1, wherein thereflecting optical element is constructed of a prism that satisfies thefollowing condition (4): 1.55<n_(pn) . . . (4) where n_(pri) is a d-linerefractive index of a medium of the prism in the first lens group. 10.The zoom lens according to claim 1, which satisfies the followingcondition (a): 1.8<f _(T) /F _(W)  (a) where f_(W) is a focal length ofthe zoom lens at a wide-angle end thereof, and f_(T) is a focal lengthof the zoom lens at a telephoto end thereof.
 11. The zoom lens accordingto claim 1, wherein the first lens group has positive refracting power,and the second lens group and the third lens group satisfy the followingconditions (5), (6) and (7): 0.4<−β_(2W)<1.2  (5) 0.1<−β_(RW)<0.5  (6)0<log γ_(R)/log γ₂<1.3  (7) where β_(2W) is a magnification of thesecond lens group at a wide-angle end of the zoom lens upon focused onan infinite object point, β_(RW) is a composite magnification of acombined system comprising the third lens group and all subsequent lensgroups at the wide-angle end upon focused on an infinite object point,γ₂ is β_(2T)/β_(2W) provided that β_(2T) is a magnification of thesecond lens group at a telephoto end of the zoom lens upon focused on aninfinite object point, and γ_(R) is β_(RT)/β_(RW) provided that β_(RT)is a composite magnification of a combined system comprising the thirdlens group and all subsequent lens groups at the telephoto end uponfocused on an infinite object point.
 12. The zoom lens according toclaim 11, wherein the first lens group and the second lens group satisfythe following conditions (8) and (9): 1.6<f ₁/{square root over ( )}(f_(W) ·f _(T))<6.0  (8) 1.1<−f ₂/{square root over ( )}(f _(W) ·f_(T))<2.2  (9) where f₁ is a focal length of the first lens group, f₂ isa focal length of the second lens group, and f_(W) and f_(T) are focallengths of the zoom lens at the wide-angle end and the telephoto end ofthe zoom lens, respectively.
 13. The zoom lens according to claim 11,wherein the combined system comprising the third lens group and allsubsequent lens-groups satisfies the following condition (10): 0.8<f_(RW)/{square root over ( )}(f _(W) ·f _(T))<1.7  (10) where f_(RW) is acomposite focal length of the combined system comprising the third lensgroup and all subsequent lens groups at the wide-angle end, and f_(W)and f_(T) are the focal lengths of the zoom lens at the wide-angle endand telephoto end, respectively.
 14. The zoom lens according to claim11, wherein the third lens group has therein a converging surfacedefined by an air contact surface that is convex on an object sidethereof and satisfies the following condition (b) and a divergingsurface defined by an air contact surface that is concave on an imageside thereof and satisfies the following condition (c): 0<R _(P) /f_(W)<2  (b) 0<R _(N) /f _(W)<4  (c) where R_(P) is an axial radius ofcurvature of the converging surface, R_(N) is an axial radius ofcurvature of the diverging surface, and f_(W) is a focal length of thezoom lens at the wide-angle end.
 15. The zoom lens according to claim11, wherein the combined system comprising the third lens group and allsubsequent lens groups satisfies the following condition (11): 1.0<f_(RT) /f _(RW)<2.5  (11) where f_(RW) is a composite focal length of thecombined system comprising the third lens group and all subsequent lensgroups at the wide-angle end, and f_(RT) is a composite focal length ofthe combined system comprising the third lens group and all subsequentlens groups at the telephoto end.
 16. The zoom lens according to claim1, wherein upon focusing on an infinite object point, the third lensgroup moves nearer to the object side of the zoom lens at the telephotoend than at the wide-angle end, and the fourth lens group moves nearerto the image side of the zoom lens at the telephoto end than at thewide-angle end.
 17. The zoom lens according to claim 16, wherein thesecond lens group, the third lens group and the fourth lens groupsatisfy the following conditions (12) and (13): 0.20<−M ₃ /M₂<1.50  (12) 0.15<−M ₄ /M ₃<1.00  (13) where M₂ is an amount of movementof the second lens group from the wide-angle end to the telephoto end,M₃ is an amount of movement of the third lens group from the wide-angleend to the telephoto end, and M₄ is an amount of movement of the fourthlens group from the wide-angle end to the telephoto end, provided thatthe movement of each lens group toward the image side is of positivesign.
 18. The zoom lens according to claim 4, wherein the fourth lensgroup satisfies the following condition (14): 0.10<D _(34W) /f_(W)<0.70  (14) where D_(34w) is an air separation between the thirdlens group and the fourth lens group at the wide-angle end upon focusedon an infinite object point, and f_(W) is a focal length of the zoomlens at the wide-angle end.
 19. The zoom lens according to claim 5,wherein the third lens group comprises, in order from an object sidethereof, a doublet component consisting of a positive lens element and anegative lens element and a single lens component with both surfacesdefined by aspheric surfaces, two lens components and three lenselements in all.
 20. The zoom lens according to claim 5, wherein thethird lens group comprises, in order from an object side thereof, asingle lens component with both surfaces defined by aspheric surfacesand a doublet component consisting of a positive lens element and anegative lens element, two lens components and three lens elements inall.
 21. The zoom lens according to claim 5, wherein the third lensgroup comprises a doublet component in which, in order from an objectside of the third lens group, a positive lens element and a negativelens element are cemented together, wherein a surface nearest to theobject side of the third lens group and a surface nearest to an imageside of the third lens group are each defined by an aspheric surface.22. The zoom lens according to claim 19, wherein the doublet componentsatisfies the following condition (15-1): 1.05<R _(C3) /R_(C1)<3.00  (15-1) where R_(C1) is an axial radius of curvature of thesurface nearest to the object side of the doublet component, and R_(C3)is an axial radius of curvature of the surface nearest to the image sideof the doublet component.
 23. The zoom lens according to claim 20,wherein the doublet component satisfies the following condition (15-2):0.25<R _(C3) /R _(C1)<0.75  (15-2) where R_(C1) is an axial radius ofcurvature of the surface nearest to the object side of the doubletcomponent, and R_(C3) is an axial radius of curvature of the surfacenearest to the image side of the doublet component.
 24. The zoom lensaccording to claim 6, wherein the third lens group comprises, in orderfrom an object side thereof, a doublet component consisting of apositive lens element and a negative lens element and a single lenscomponent with both surfaces defined by aspheric surfaces, two lenscomponents and three lens elements in all.
 25. The zoom lens accordingto claim 6, wherein the third lens group comprises, in order from anobject side thereof, a single lens component with both surfaces definedby aspheric surfaces and a doublet component consisting of a positivelens element and a negative lens element, two lens components and threelens elements in all.
 26. The zoom lens according to claim 6, whereinthe third lens group comprises a doublet component in which, in orderfrom an object side of the third lens group, a positive lens element anda negative lens element are cemented together, wherein a surface nearestto the object side of the third lens group and a surface nearest to animage side of the third lens group are each defined by an asphericsurface.
 27. The zoom lens according to claim 7, wherein the third lensgroup comprises, in order from an object side thereof, a doubletcomponent consisting of a positive lens element and a negative lenselement and a single lens component with both surfaces defined byaspheric surfaces, two lens components and three lens elements in all.28. The zoom lens according to claim 7, wherein the third lens groupcomprises, in order from an object side thereof, a single lens componentwith both surfaces defined by aspheric surfaces and a doublet componentconsisting of a positive lens element and a negative lens element, twolens components and three lens elements in all.
 29. The zoom lensaccording to claim 7, wherein the third lens group comprises a doubletcomponent in which, in order from an object side of the third lensgroup, a positive lens element and a negative lens element are cementedtogether, wherein a surface nearest to the object side of the third lensgroup and a surface nearest to an image side of the third lens group areeach defined by an aspheric surface.
 30. The zoom lens according toclaim 19, wherein the doublet component satisfies the followingcondition (15-3): 1.20<R _(C3) /R _(C1)<3.60  (15-3) where R_(C1) is anaxial radius of curvature of the surface nearest to the object side ofthe doublet component, and R_(C3) is an axial radius of curvature of thesurface nearest to the image side of the doublet component.
 31. The zoomlens according to claim 1, wherein the fourth lens group comprises onepositive lens component.
 32. The zoom lens according to claim 1, whereinthe second lens group comprises, in order from an object side thereof, anegative lens component and a positive lens component, two lenscomponents in all.
 33. The zoom lens according to claim 32, wherein thefirst lens group comprises a positive lens component on an image side ofthe reflecting optical element, and the positive lens component and thesecond lens group satisfy the following conditions (20) and (21):−0.80<(R _(1PF) +R _(1PR))/(R _(1PF) −R _(1PR))<0.90  (20) −0.10<(R_(2NF) +R _(2NR))/(R _(2NF) −R _(2NR))<2.00  (21) where R_(1PF) is anaxial radius of curvature of an object side-surface of the positive lenscomponent in the first lens group, R_(1PR) is an axial radius ofcurvature of an image side-surface of the positive lens component in thefirst lens group, R_(2NF) is an axial radius of curvature of an objectside-surface of the negative lens component in the second lens group,and R_(2NR) is an axial radius of curvature of an image side-surface ofthe negative lens component in the second lens group.
 34. An electronicimaging system, comprising a zoom lens as recited in claim 1, and anelectronic image pickup device located on an image side of the zoomlens.
 35. An electronic imaging system, comprising a zoom lens asrecited in claim 1 and an electronic image pickup device located on animage side of the zoom lens, wherein the zoom lens satisfies thefollowing condition (3): 0.8<d/L<2.0  (3) where d is an air-based lengthfrom an image side-surface of the negative meniscus lens component inthe first lens group to an object side-surface of the positive lenscomponent in the first lens group, as measured along an optical axis ofthe zoom lens, and L is a diagonal length of an effective image pickuparea of the electronic image pickup device.
 36. An electronic imagingsystem, comprising a zoom lens as recited in claim 19 and an electronicimage pickup device located on an image side of the zoom lens, whereinthe zoom lens satisfies the following conditions (16-1) and (17-1):−0.7<L/R _(C2)<0.1  (16-1) 10<ν_(CP)−ν_(CN)  (17-1) where L is adiagonal length of an effective image pickup area of the electronicimage pickup device, R_(C2) is an axial radius of curvature of acementing surface of the doublet component in the third lens group,ν_(CP) is a d-line based Abbe number of a medium of the positive lenselement of the doublet component in the third lens group, and ν_(CN) isa d-line based Abbe number of a medium of the negative lens element ofthe doublet component in the third lens group.
 37. An electronic imagingsystem, comprising a zoom lens as recited in claim 20 and an electronicimage pickup device located on an image side of the zoom lens, whereinthe zoom lens satisfies the following conditions (16-2) and (17-2):−0.5<L/R _(C2)<0.3  (16-2) 20<ν_(CP)−ν_(CN)  (17-2) where L is adiagonal length of an effective image pickup area of the electronicimage pickup device, R_(C2) is an axial radius of curvature of acementing surface of the doublet component in the third lens group,ν_(CP) is a d-line based Abbe number of a medium of the positive lenselement of the doublet component in the third lens group, and ν_(CN) isa d-line based Abbe number of a medium of the negative lens element ofthe doublet component in the third lens group.
 38. An electronic imagingsystem, comprising a zoom lens as recited in claim 29 and an electronicimage pickup device located on an image side of the zoom lens, whereinthe zoom lens satisfies the following conditions (16-3) and (17-3):−0.9<L/R _(C2)<−0.1  (16-3) 10<ν_(CP)−ν_(CN)  (17-3) where L is adiagonal length of an effective image pickup area of the electronicimage pickup device, R_(C2) is an axial radius of curvature of acementing surface of the doublet component in the third lens group,ν_(CP) is a d-line based Abbe number of a medium of the positive lenselement of the doublet component in the third lens group, and ν_(CN) isa d-line based Abbe number of a medium of the negative lens element ofthe doublet component in the third lens group.
 39. An electronic imagingsystem, comprising a zoom lens as recited in claim 31 and an electronicimage pickup device located on an image side of the zoom lens, whereinthe positive lens component in the fourth lens group of the zoom lenssatisfies the following conditions (18) and (19): −4.00<(R _(4F) +R_(4R))/(R _(4F) −R _(4R))<0.0  (18) 0.10<L/f ₄<0.70  (19) where R_(4F)is an axial radius of curvature of an object side-surface of thepositive lens component, R_(4R) is an axial radius of curvature of animage side-surface of the positive lens component, L is a diagonallength of an effective image pickup area of the electronic image pickupdevice, and f₄ is a focal length of the fourth lens group.
 40. Theelectronic imaging system according to claim 39, which has a total angleof view of 55° or greater at the wide-angle end.
 41. The electronicimaging system according to claim 40, which has a total angle of view of80° or smaller at the wide-angle end.